x86/emul: Provide a wrapper to x86_emulate() to ASSERT() certain behaviour

x86/emul: Provide a wrapper to x86_emulate() to ASSERT() certain behaviour

In debug builds, confirm that some properties of x86_emulate()'s behaviour
actually hold.  The first property, fixed in a previous change, is that retire
flags are only ever set in the X86EMUL_OKAY case.

While adjusting the userspace test harness to cope with ASSERT() in
x86_emulate.h, fix a build problem introduced in c/s 122dd9575c7 "x86emul:
in_longmode() should not ignore ->read_msr() errors" by providing an
implementation of likely()/unlikely().

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