KVM: x86 emulator: Rename emulate_grpX() to em_grpX()

The prototypes are changed appropriately.

We also replaces "goto grp45;" with simple em_grp45() call.

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