KVM: x86 emulator: Place insn_fetch helpers together

The two macros need special care to use:
  Assume rc, ctxt, ops and done exist outside of them.
  Can goto outside.

Considering the fact that these are used only in decode functions,
moving these right after do_insn_fetch() seems to be a right thing
to improve the readability.

We also rename do_fetch_insn_byte() to do_insn_fetch_byte() to be
consistent.

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