virt: Adds possibility to choose different data directory from params.

param:
  images_base_dir = "new path"

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds migration tests with restart NFS, iSCSI server.

iSCSI test supports only tgtd implementation. But there is prepared
interface for adding other type of iSCSI server.

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