x86/emul: Remove opencoded exception generation

x86/emul: Remove opencoded exception generation

Introduce generate_exception() for unconditional exception generation, and
replace existing uses.  Both generate_exception() and generate_exception_if()
are updated to make their error code parameters optional, which removes the
use of the -1 sentinal.

The ioport_access_check() check loses the presence check for %tr, as the x86
architecture has no concept of a non-usable task register.

No functional change.

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