KVM: x86 emulator: Use opcode::execute for PUSHA/POPA (60/61)

For this, emulate_pusha/popa() are converted to em_pusha/popa().

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