x86emul: support MMX/SSE{,2,3} moves

x86emul: support MMX/SSE{,2,3} moves

Previously supported insns are being converted to the new model, and
several new ones are being added.

To keep the stub handling reasonably simple, integrate SET_SSE_PREFIX()
into copy_REX_VEX(), at once switching the stubs to use an empty REX
prefix instead of a double DS: one (no byte registers are being
accessed, so an empty REX prefix has no effect), except (of course) for
the 32-bit test harness build.

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