KVM: x86: Sysexit emulation does not mask RIP/RSP

If the operand size is not 64-bit, then the sysexit instruction should assign
ECX to RSP and EDX to RIP.  The current code assigns the full 64-bits.

Fix it by masking.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>