virDomainVideoDefParseXML: Fix parsing of 'backend'
virDomainVideoDefParseXML: Fix parsing of 'backend'

Commit 8391cfbc2dbc converted the code to use virXMLPropEnum unfaithfully
ommitting the check where 'backend' must be non-zero when parsed from the
user.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt