The virTimestamp and virTimeMs functions in src/util/util.h
duplicate functionality from virtime.h, in a non-async signal
safe manner. Remove them, and convert all code over to the new
APIs.

* src/util/util.c, src/util/util.h: Delete virTimeMs and virTimestamp
* src/lxc/lxc_driver.c, src/qemu/qemu_domain.c,
  src/qemu/qemu_driver.c, src/qemu/qemu_migration.c,
  src/qemu/qemu_process.c, src/util/event_poll.c: Convert to use
  virtime APIs