Instead of relying solely on polling for /dev devices to appear in libvirt, we

Instead of relying solely on polling for /dev devices to appear in libvirt, we
really should be synchronizing against udev.  This is generally done by a call
to udevsettle, which is exactly what this patch implements for the storage
backends that are likely to create new /dev nodes.  I believe I've read that
even after udevsettle, you are not guaranteed that devices are all the way
created, so we still need the polling in the rest of the sources, but this
should give us a much better chance of things existing as we expect.

Signed-off-by: Chris Lalancette <clalance@redhat.com>