virStorageSourceGetBackingStoreStr: Move the function earlier
virStorageSourceGetBackingStoreStr: Move the function earlier

Move it together with virStorageSourceGetRelativeBackingPath which is
the main reason why it exists. Upcoming patch will modify the comment
and arguments refering to virStorageSourceGetRelativeBackingPath so it's
better if they are together.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

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