KVM: x86 emulator: Use opcode::execute for XCHG(86/87)

In addition, replace one "goto xchg" with an em_xchg() call.

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