conf: domain: Introduce an internal variant of virDomainDiskDefNew
conf: domain: Introduce an internal variant of virDomainDiskDefNew

The <disk> XML element parser is going to be modified so that the
virStorageSource bits are pre-parsed. Add virDomainDiskDefNewSource,
which uses an existing 'src' pointer.

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

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