Commit 1d22ba95 was complete at the time, but we have since
reintroduced a warning that is fixed in the same manner:

  CCLD   storagebackendsheepdogtest

*** Warning: Linking the executable storagebackendsheepdogtest against the loadable module
*** libvirt_driver_storage.so is not portable!

* src/Makefile.am (libvirt_driver_storage.la): Factor into new
convenience library libvirt_driver_storage_impl.la.
* tests/Makefile.am (storagebackendsheepdogtest_LDADD): Link to
convenience library, not shared library.