x86/emul: Split exception handling out of invoke_stub()

x86/emul: Split exception handling out of invoke_stub()

For a release build, bloat-o-meter reports:

  add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-5111 (-5111)
  function                                     old     new   delta
  x86_emulate                               126458  121347   -5111

or in other words, a 4% redunction in code size from this change alone.

The use of __LINE__ is a concern with livepatching, but any livepatch touching
this file is overwhemlingly likely to alter x86_emulate() anyway.

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