Create a new set of sub-packages containing the new storage driver
modules so that certain heavy-weight backends (gluster, rbd) can be
installed separately only if required.

To keep backward compatibility the 'libvirt-driver-storage' package
will be turned into a virtual package pulling in all the new storage
backend sub-packages. The storage driver module will be moved into
libvirt-driver-storage-core including the filesystem backend which is
mandatory.

This then allows to make libvirt-daemon-driver-qemu depend only on the
core of the storage driver.

All other meta-packages still depend on the full storage driver and thus
pull in all the backends.