x86: miscellaneous emulator adjustments

x86: miscellaneous emulator adjustments

Defer fail_if()-s as much as possible (in favor of possibly generating
exceptions), and avoid generating exceptions when not strictly
necessary.

Avoid fail_if()-s for simple return code checks (making the code that
used them consistent with other, longer existing code).

Eliminate redundant generate_exception_if()-s checking lock_prefix
(which is already covered by the general check prior to decoding
operands).

Also fix the testing code to add PROT_EXEC for the mapping that is
intended to have instruction executed from.

Signed-off-by: Jan Beulich <jbeulich@novell.com>