x86/vvmx: Fix deadlock with MSR bitmap merging

x86/vvmx: Fix deadlock with MSR bitmap merging

c/s c47984aabead "nvmx: implement support for MSR bitmaps" introduced a use of
map_domain_page() which may get used in the middle of context switch.

This is not safe, and causes Xen to deadlock on the mapcache lock:

  (XEN) Xen call trace:
  (XEN)    [<ffff82d08022d6ae>] R _spin_lock+0x34/0x5e
  (XEN)    [<ffff82d0803219d7>] F map_domain_page+0x250/0x527
  (XEN)    [<ffff82d080356332>] F do_page_fault+0x420/0x780
  (XEN)    [<ffff82d08038da3d>] F x86_64/entry.S#handle_exception_saved+0x68/0x94
  (XEN)    [<ffff82d08031729f>] F __find_next_zero_bit+0x28/0x69
  (XEN)    [<ffff82d080321a4d>] F map_domain_page+0x2c6/0x527
  (XEN)    [<ffff82d08029eeb2>] F nvmx_update_exec_control+0x1d7/0x323
  (XEN)    [<ffff82d080299f5a>] F vmx_update_cpu_exec_control+0x23/0x40
  (XEN)    [<ffff82d08029a3f7>] F arch/x86/hvm/vmx/vmx.c#vmx_ctxt_switch_from+0xb7/0x121
  (XEN)    [<ffff82d08031d796>] F arch/x86/domain.c#__context_switch+0x124/0x4a9
  (XEN)    [<ffff82d080320925>] F context_switch+0x154/0x62c
  (XEN)    [<ffff82d080252f3e>] F common/sched/core.c#sched_context_switch+0x16a/0x175
  (XEN)    [<ffff82d080253877>] F common/sched/core.c#schedule+0x2ad/0x2bc
  (XEN)    [<ffff82d08022cc97>] F common/softirq.c#__do_softirq+0xb7/0xc8
  (XEN)    [<ffff82d08022cd38>] F do_softirq+0x18/0x1a
  (XEN)    [<ffff82d0802a2fbb>] F vmx_asm_do_vmentry+0x2b/0x30

Convert the domheap page into being a xenheap page.

Fixes: c47984aabead - nvmx: implement support for MSR bitmaps
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>