KVM: x86: stubs for SMM support

This patch adds the interface between x86.c and the emulator: the
SMBASE register, a new emulator flag, the RSM instruction.  It also
adds a new request bit that will be used by the KVM_SMI ioctl.

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>