rbd: Remove snapshots if the DELETE_WITH_SNAPSHOTS flag has been provided

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>