x86/pv: Fix crash when using `xl set-parameter pcid=...`

x86/pv: Fix crash when using `xl set-parameter pcid=...`

"pcid=" is registered as a runtime parameter, which means that parse_pcid()
must not reside in .init, or the following happens when parse_params() tries
to call an unmapped function pointer.

  (XEN) ----[ Xen-4.12-unstable  x86_64  debug=y   Not tainted ]----
  (XEN) CPU:    0
  (XEN) RIP:    e008:[<ffff82d080407fb3>] ffff82d080407fb3
  (XEN) RFLAGS: 0000000000010292   CONTEXT: hypervisor (d0v1)
  (XEN) rax: ffff82d080407fb3   rbx: ffff82d0803cf270   rcx: 0000000000000000
  (XEN) rdx: ffff8300abe67fff   rsi: 000000000000000a   rdi: ffff8300abe67bfd
  (XEN) rbp: ffff8300abe67ca8   rsp: ffff8300abe67ba0   r8:  ffff83084d980000
  (XEN) r9:  0000000000000000   r10: 0000000000000000   r11: 0000000000000000
  (XEN) r12: ffff8300abe67bfd   r13: ffff82d0803cb628   r14: 0000000000000000
  (XEN) r15: ffff8300abe67bf8   cr0: 0000000080050033   cr4: 0000000000172660
  (XEN) cr3: 0000000828efd000   cr2: ffff82d080407fb3
  (XEN) fsb: 00007fb810d4b780   gsb: ffff88007ce20000   gss: 0000000000000000
  (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e010   cs: e008
  (XEN) Xen code around <ffff82d080407fb3> (ffff82d080407fb3) [fault on access]:
  (XEN)  -- -- -- -- -- -- -- -- <--> -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
  (XEN) Xen stack trace from rsp=ffff8300abe67ba0:
  (XEN)    ffff82d080217f61 ffff830826db0f09 ffff8300abe67bf8 ffff82d0803cf1e0
  (XEN)    00007cff54198409 ffff8300abe67bf0 010001d000000000 0000000000000000
  (XEN)    ffff82d0803cf288 ffff8300abe67c88 ffff82d0805a09c0 616c620064696370
  (XEN)    00000000aaaa0068 0000000000000296 ffff82d08023d60e aaaaaaaaaaaaaaaa
  (XEN)    ffff83084d9b4000 ffff8300abe67c68 ffff82d08024940e ffff83083736e000
  (XEN)    0000000000000080 000000000000007a 000000000000000a ffff82d08045e61c
  (XEN)    ffff82d080573d80 ffff8300abe67cb8 ffff82d080249805 80000007fce54067
  (XEN)    fffffffffffffff2 ffff830826db0f00 ffff8300abfa7000 ffff82d08045e61c
  (XEN)    ffff82d080573d80 ffff8300abe67cb8 ffff82d08021801e ffff8300abe67e48
  (XEN)    ffff82d08023f60a ffff83083736e000 0000000000000000 ffff8300abe67d58
  (XEN)    ffff82d080293d90 0000000000000092 ffff82d08023d60e ffff820040006ae0
  (XEN)    0000000000000000 0000000000000000 00007fb810d5c010 ffff83083736e248
  (XEN)    0000000000000286 ffff8300abe67d58 0000000000000000 ffff82e010521b00
  (XEN)    0000000000000206 0000000000000000 0000000000000000 ffff8300abe67e48
  (XEN)    ffff82d080295270 00000000ffffffff ffff83083736e000 ffff8300abe67e48
  (XEN)    ffff820040006ae0 ffff8300abe67d98 000000120000001c 00007fb810d5d010
  (XEN)    0000000000000009 0000000000000002 0000000000000001 00007fb810b53260
  (XEN)    0000000000000001 0000000000000000 0000000000638bc0 00007fb81066a748
  (XEN)    00007ffe11087881 0000000000000002 0000000000000001 00007fb810b53260
  (XEN)    0000000000638b60 0000000000000000 00007fb8100322a0 ffff82d08035d444
  (XEN) Xen call trace:
  (XEN)    [<ffff82d080217f61>] kernel.c#parse_params+0x34a/0x3eb
  (XEN)    [<ffff82d08021801e>] runtime_parse+0x1c/0x1e
  (XEN)    [<ffff82d08023f60a>] do_sysctl+0x108d/0x1241
  (XEN)    [<ffff82d0803535cb>] pv_hypercall+0x1ac/0x4c5
  (XEN)    [<ffff82d08035d4a2>] lstar_enter+0x112/0x120
  (XEN)
  (XEN) Pagetable walk from ffff82d080407fb3:
  (XEN)  L4[0x105] = 00000000abe5c063 ffffffffffffffff
  (XEN)  L3[0x142] = 00000000abe59063 ffffffffffffffff
  (XEN)  L2[0x002] = 000000084d9bf063 ffffffffffffffff
  (XEN)  L1[0x007] = 0000000000000000 ffffffffffffffff
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 0:
  (XEN) FATAL PAGE FAULT
  (XEN) [error_code=0010]
  (XEN) Faulting linear address: ffff82d080407fb3
  (XEN) ****************************************

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>