KVM: x86 emulator: Fix handling of VMMCALL instruction

The VMMCALL instruction doesn't get recognised and isn't processed
by the emulator.

This is seen on an Intel host that tries to execute the VMMCALL
instruction after a guest live migrates from an AMD host.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>