vbox: fix a segfault when taking a snapshot
vbox: fix a segfault when taking a snapshot

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 bc3f5f1
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: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

elmarco/libvirt,taget/libvirt,taget/libvirt,jfehlig/libvirt,elmarco/libvirt,zippy2/libvirt,eskultety/libvirt,eskultety/libvirt,agx/libvirt,rlaager/libvirt,eskultety/libvirt,eskultety/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,datto/libvirt,nertpinx/libvirt,libvirt/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,shugaoye/libvirt,jfehlig/libvirt,elmarco/libvirt,rlaager/libvirt,libvirt/libvirt,agx/libvirt,nertpinx/libvirt,andreabolognani/libvirt,agx/libvirt,agx/libvirt,olafhering/libvirt,VenkatDatta/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,datto/libvirt,rlaager/libvirt,crobinso/libvirt,shugaoye/libvirt,olafhering/libvirt,VenkatDatta/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,datto/libvirt,datto/libvirt,rlaager/libvirt,elmarco/libvirt,taget/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,elmarco/libvirt,shugaoye/libvirt,taget/libvirt,jfehlig/libvirt,agx/libvirt,crobinso/libvirt,nertpinx/libvirt,jfehlig/libvirt,rlaager/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,eskultety/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,taget/libvirt,zippy2/libvirt,olafhering/libvirt,zippy2/libvirt,datto/libvirt