xci: scripts: vm: Use LVM backend if available

xci: scripts: vm: Use LVM backend if available

The CI nodes come with a big secondary mechanical disk which can be
attached directly to the clean VM. As such, we split the disk into one
logical volume per distribution and flush the qcow2 image to it before
attaching it to the VM. If the XCI volume group is not available, then
we revert back to using a file-based storage backend. Finally, we hide
stdout/stderr from the package installation process to minimize the
script output.

Change-Id: I11ea82899c411f8a4168e5f23cbbd8187cc891df
Signed-off-by: Markos Chandras <mchandras@suse.de>