virt: Adds support for filesystem backend to storage.

backend could be glusterfs, nfs, ...
Only gluster fs implemented now.
Necessary for gluster testing.
virt: gluster split create and get path functionality.

It should be possible get gluster volume path without create
gluster volume.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds support for qcow2 backing file.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds support for check disk in storage with native support (gluster).

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: env_preprocess adds support for image backend.

Gluster and iSCSI storage are special because it is supported
native in qemu-kvm different way from other storage backend.

-drive gluster://server1:0/vol_test/f19-64.qcow2
-drive iscsi://<target-ip>[:<port>]/<target-iqn>/<lun>

other backends are firstly connected to system filesystem and
then used through the file path in VM like normal file or
uses other method like p9.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Change config files to use image_backend config.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds some gluster tests.

Tests:
1) boot_snap_boot.
2) gluster image create test.

Signed-off-by: Jiří Župka <jzupka@redhat.com>