x86/HVM: replace plain number in hvm_combine_hw_exceptions()

x86/HVM: replace plain number in hvm_combine_hw_exceptions()

While doing so also take care of #VE here (even if we don't make use of
it yet). Note that contributory_exceptions, other than the original
0x7c01 constant, doesn't include #PF anymore, but the check where the
variable is used is after one that already filtered out #PF.

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