x86emul: fix pushing of selector registers

x86emul: fix pushing of selector registers

Both explicit PUSH and far CALL currently push unrelated data (the
segment attributes word) in the high half (attributes and limit in the
64-bit case in the high 48 bits) instead of zero. To avoid having to
apply this and further changes in multiple places, also fold the two
(respectively) far call/jmp instances into one.

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