KVM: x86 emulator: emulate clflush

If the guest issues a clflush in a mmio address, the instruction
can trap into the hypervisor. Currently, we do not decode clflush
properly, causing the guest to hang. This patch fixes this emulating
clflush (opcode 0f ae).

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>