Commit 1c275e9a accidentally dropped the storage driver from
libvirtd, because it depended on a C preprocessor macro that
was not defined.  Furthermore, if you do './configure
--without-storage-dir --with-storage-disk' or any other combination
where you explicitly build a subset of storage backends excluding
the dir backend, then the build is broken.

Based on analysis by Osier Yang.

* configure.ac (WITH_STORAGE): Define top-level conditional.
* src/Makefile.am (mod_LTLIBRARIES): Build driver even when
storage_dir is disabled.
* daemon/libvirtd.c: Pick up storage driver for any backend, not
just dir.
* daemon/Makefile.am (libvirtd_LDADD): Likewise.