KVM: x86 emulator: Use opcode::execute for Group 1A instruction

Group 1A: 8F

Register em_pop() directly and remove em_grp1a().

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>