We introduce em_jmp_far().

We also call this from em_grp45() to stop treating modrm_reg == 5 case
separately in the group 5 emulation.
Signed-off-by: NTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: NAvi Kivity <avi@redhat.com>