KVM: x86: pop sreg accesses only 2 bytes

Although pop sreg updates RSP according to the operand size, only 2 bytes are
read.  The current behavior may result in incorrect #GP or #PF exceptions.

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