storage : vdisk_free : eval parse_volname

If the storage have some customs volumes name, vdisk_free will hang

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
vmid is integer an can be 0, so we need to use defined() to check if it is set

Note: The plan is to use vmid 0 to mark shared volumes.
rbd : parse_volname : parse base

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
rbd : rbd_ls : parse base

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>