KVM: x86: POP [ESP] is not emulated correctly

According to Intel SDM: "If the ESP register is used as a base register for
addressing a destination operand in memory, the POP instruction computes the
effective address of the operand after it increments the ESP register."

The current emulation does not behave so. The fix required to waste another
of the precious instruction flags and to check the flag in decode_modrm.

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