KVM: x86 emulator: Use opcode::execute for some instructions

Move the following functions to the opcode tables:

  RET (Far return) : CB
  IRET             : CF
  JMP (Jump far)   : EA

  SYSCALL          : 0F 05
  CLTS             : 0F 06
  SYSENTER         : 0F 34
  SYSEXIT          : 0F 35

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