This patch changes virFileLoopDeviceOpen() to use the new loop-control
device to allocate a new loop device.  If this behavior is unsupported
we fall back to the previous method of searching /dev for a free device.

With this patch you can start as many image based LXC domains as you
like (well almost).

Fixes bug https://bugzilla.redhat.com/show_bug.cgi?id=995543