Provide support for this insn, which is a prereq to FRED. CPUID-wise,
while its and FRED's enumerators were already introduced, their dependency
still needs adding.
While adding a testcase, also add a SWAPGS one. In order to not affect
the behavior of pre-existing tests, install write_{segment,msr} hooks
only transiently.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>