x86: set regs->entry_vector for early_page_fault

x86: set regs->entry_vector for early_page_fault

This changes:

(XEN) Early fatal page fault at e008:ffff82d080164252 (cr2=0000000000000000, ec=0000)
(XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d080164252>] arch_domain_create+0x3e/0x4ef
...
(XEN) Xen call trace:
(XEN)    [<ffff82d080164252>] arch_domain_create+0x3e/0x4ef
(XEN)    [<ffff82d080105262>] domain_create+0x384/0x556
(XEN)    [<ffff82d0802a0de4>] scheduler_init+0x1c4/0x244
(XEN)    [<ffff82d0802be359>] __start_xen+0x1d0e/0x22a1
(XEN)    [<ffff82d080100067>] __high_start+0x53/0x58
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) FATAL TRAP: vector = 0 (divide error)
(XEN) [error_code=0000] , IN INTERRUPT CONTEXT
(XEN) ****************************************
...

to:

(XEN) Early fatal page fault at e008:ffff82d080164252 (cr2=0000000000000000, ec=0000)
(XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d080164252>] arch_domain_create+0x3e/0x4ef
...
(XEN) Xen call trace:
(XEN)    [<ffff82d080164252>] arch_domain_create+0x3e/0x4ef
(XEN)    [<ffff82d080105262>] domain_create+0x384/0x556
(XEN)    [<ffff82d0802a0de4>] scheduler_init+0x1c4/0x244
(XEN)    [<ffff82d0802be359>] __start_xen+0x1d0e/0x22a1
(XEN)    [<ffff82d080100067>] __high_start+0x53/0x58
(XEN)
(XEN) Faulting linear address: 0000000000000000
(XEN) Pagetable walk from 0000000000000000:
(XEN)  L4[0x000] = 000000083a1a6063 ffffffffffffffff
(XEN)  L3[0x000] = 000000083a1a5063 ffffffffffffffff
(XEN)  L2[0x000] = 000000083a1a4063 ffffffffffffffff
(XEN)  L1[0x000] = 0000000000000000 ffffffffffffffff
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) FATAL TRAP: vector = 14 (page fault)
(XEN) [error_code=0000] , IN INTERRUPT CONTEXT
(XEN) ****************************************
...

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>