x86/hvm: Increase the triple fault log message level to XENLOG_ERR

x86/hvm: Increase the triple fault log message level to XENLOG_ERR

At INFO level, it doesn't get printed out by default in release builds,
leading to unqualified logging such as this:

  (XEN) [   66.995993] Freed 524kB init memory
  (XEN) [ 1993.144997] *** Dumping Dom9 vcpu#2 state: ***
  (XEN) [ 1993.145008] ----[ Xen-4.11.1  x86_64  debug=n   Not tainted ]----
  (XEN) [ 1993.145011] CPU:    21
  (XEN) [ 1993.145015] RIP:    0010:[<ffffe0002ba950ef>]
  (XEN) [ 1993.145018] RFLAGS: 0000000000010246   CONTEXT: hvm guest (d9v2)
  (XEN) [ 1993.145026] rax: 00000000ffffe000   rbx: ffffe0002d8e1440   rcx: 0000ffffe0002ba9
  (XEN) [ 1993.145031] rdx: 0000000000000000   rsi: ffffe0002ba93575   rdi: fffff803dfb9f340
  (XEN) [ 1993.145035] rbp: ffffd001cd791200   rsp: ffffd001cd791140   r8:  0000000000000130
  (XEN) [ 1993.145039] r9:  0000000080000000   r10: 0000000000000000   r11: 0000000000000020
  (XEN) [ 1993.145043] r12: ffffe0002ba9306d   r13: 0000000000000000   r14: 0000000000000001
  (XEN) [ 1993.145047] r15: fffff803dfb9f200   cr0: 0000000080050031   cr4: 0000000000170678
  (XEN) [ 1993.145051] cr3: 00000000001aa002   cr2: 0000020488403f70
  (XEN) [ 1993.145056] fsb: 0000000060f71000   gsb: ffffd001cc1af000   gss: 0000009d60f6f000
  (XEN) [ 1993.145060] ds: 002b   es: 002b   fs: 0053   gs: 002b   ss: 0018   cs: 0010

A triple fault is fatal to the domain under all circumstances (so will print
at most once), and in practice is always an error condition rather than a
reboot fallback.

Reported-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>