coverity: Model g_poll()

In my testing, Coverity reported two more CHECKED_RETURN:

* qemu-char.c:1248: fixed in commit c1f2448: "qemu-char: retry g_poll
  on EINTR".

* migration/qemu-file-unix.c:75: harmless, cleaned up in commit
  4e39f57 "migration: Clean up use of g_poll() in
  socket_writev_buffer()

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1450336833-27710-1-git-send-email-armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
