KVM: x86 emulator: Use opcode::execute for LOOP/JCXZ

  LOOP/LOOPcc      : E0-E2
  JCXZ/JECXZ/JRCXZ : E3

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>