docs: schemas: Fix missing timestamp inside backingStore
docs: schemas: Fix missing timestamp inside backingStore

All backingStore XML definitions have a XML tag with the timestamp. This
timestamp is not defined insinde RNG volume storage schema and it is
causing some problems to validate and check volume XMLs.

https://bugzilla.redhat.com/show_bug.cgi?id=1594266

Signed-off-by: Julio Faracco <a9e27ab562b8d02c0041eb70cde6fe3858219fb7@gmail.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>

zippy2/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,eskultety/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,jfehlig/libvirt,eskultety/libvirt,andreabolognani/libvirt,libvirt/libvirt,libvirt/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,eskultety/libvirt,fabianfreyer/libvirt,crobinso/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt