x86/hvm: print register state upon triple fault

x86/hvm: print register state upon triple fault

A sample looks like:

(XEN) d1v0 Triple fault - invoking HVM shutdown action 1
(XEN) *** Dumping Dom1 vcpu#0 state: ***
(XEN) ----[ Xen-4.7-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    2
(XEN) RIP:    0000:[<0000000000100005>]
(XEN) RFLAGS: 0000000000010002   CONTEXT: hvm guest (d1v0)
(XEN) rax: 0000000000000020   rbx: 0000000000000000   rcx: 0000000000000000
(XEN) rdx: 0000000000000000   rsi: 0000000000000000   rdi: 0000000000000000
(XEN) rbp: 0000000000000000   rsp: 0000000000000000   r8:  0000000000000000
(XEN) r9:  0000000000000000   r10: 0000000000000000   r11: 0000000000000000
(XEN) r12: 0000000000000000   r13: 0000000000000000   r14: 0000000000000000
(XEN) r15: 0000000000000000   cr0: 0000000000000011   cr4: 0000000000000000
(XEN) cr3: 0000000000000000   cr2: 0000000000000000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: 0000

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