devices: filesystems: added volume type

devices: filesystems: added volume type

New type of <devices> <filesystem type= 'volume'> is introduced.
This patch allows to use volumes for storing the filesystem, that is
accessed from the guest e.g. root directory for container.

To take advantage of volumes as a backend of filesystem volume
and pool names should be specified:
<filesystem type= 'volume'>
 <source pool='pool name' volume='volume name'/>

Signed-off-by: Olga Krishtal <okrishtal@virtuozzo.com>