KVM: x86 emulator: Use opcode::execute for PUSHF/POPF (9C/9D)

For this, em_pushf/popf() are introduced.

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