storage: Turn storage backends into dynamic modules

storage: Turn storage backends into dynamic modules

If driver modules are enabled turn storage driver backends into
dynamically loadable objects. This will allow greater modularity for
binary distributions, where heavyweight dependencies as rbd and gluster
can be avoided by selecting only a subset of drivers if the rest is not
necessary.

The storage modules are installed into 'LIBDIR/libvirt/storage-backend/'
and users can override the location by using
'LIBVIRT_STORAGE_BACKEND_DIR' environment variable.

rpm based distros will at this point install all the backends when
libvirt-daemon-driver-storage package is installed.