Rename four functions in preparation for new API.

Rename qemu_timer_expired to timer_expired
Rename qemu_timer_expire_time_ns to timer_expire_time_ns
Rename qemu_timer_pending to timer_pending
Rename qemu_timer_expired_ns to timer_expired_ns
Signed-off-by: NAlex Bligh <alex@alex.org.uk>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>