KVM: x86 emulator: vendor specific instructions

Mark some instructions as vendor specific, and allow the caller to request
emulation only of vendor specific instructions.  This is useful in some
circumstances (responding to a #UD fault).

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>