KVM: x86 emulator: Clean up init_emulate_ctxt()

Use a local pointer to the emulate_ctxt for simplicity.  Then, arrange
the hard-to-read mode selection lines neatly.

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