there is a segfault in the vbox driver when taking a snapshot in the
following functions:
- vboxDomainGetXMLDesc
- vboxSnapshotGetReadWriteDisks
- vboxSnapshotGetReadOnlyDisks

The virStorageSourcePtr in virDomainDiskDef was not correctly allocated.

(The problem stems from the fact that commit 4dc5d8f and commit bc3f5f19
were written in one order but applied in another; so each tested in
isolation passed, but the combination introduces the problem due to
changed semantics).
Signed-off-by: NEric Blake <eblake@redhat.com>