UUID of LXC container in libvirt environment

Libvirt on container start sets some environment variables, especially
LIBVIRT_LXC_UUID, which may be used for linking container to host.

closes: #2311
move and test _getLibvirtLXC_UUID()

move _getLibvirtLXC_UUID in Virtualization/Vmsystem.pm and add
a test.

Thanks Alexander Evseev for the patch.