rbd: Remove snapshots if the DELETE_WITH_SNAPSHOTS flag has been provided
    
    When a RBD volume has snapshots it can not be removed.
    
    This patch introduces a new flag to force volume removal,
    VIR_STORAGE_VOL_DELETE_WITH_SNAPSHOTS.
    
    With this flag any existing snapshots will be removed prior to
    removing the volume.
    
    No existing mechanism in libvirt allowed us to pass such information,
    so that's why a new flag was introduced.
    
    Signed-off-by: Wido den Hollander <wido@widodh.nl>