x86/traps: Lift all non-entrypoint logic in entry_int82() up into C

x86/traps: Lift all non-entrypoint logic in entry_int82() up into C

This is more readable, maintainable, and livepatchable.

This involves declaring check_for_unexpected_msi(), untrusted_msi and
pv_hypercall() suitably for use by C.  While making these changes,
untrusted_msi is switched over to being a C99 bool.

No behavioural change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>