x86/pv: Rework guest_io_okay() to return X86EMUL_*

x86/pv: Rework guest_io_okay() to return X86EMUL_*

In order to fix a bug with guest_io_okay() (subsequent patch), rework
guest_io_okay() to take in an emulation context, and return X86EMUL_* rather
than a boolean.

For the failing case, take the opportunity to inject #GP explicitly, rather
than returning X86EMUL_UNHANDLEABLE.  There is a logical difference between
"we know what this is, and it's #GP", vs "we don't know what this is".

There is no change in practice as emulation is the final step on general #GP
resolution, but returning X86EMUL_UNHANDLEABLE would be a latent bug if a
subsequent action were to appear.

No practical change.

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