Libtool is picky about linking against a module library (aka a .so);
giving lots of warnings like this in the tests directory:

  CCLD   networkxml2argvtest

*** Warning: Linking the executable networkxml2argvtest against the loadable module
*** libvirt_driver_network.so is not portable!

Fix that by splitting things into a convenience library which can
be used directly by the tests, and making the real .so just wrap
the convenience library.

Based on a suggestion by Daniel P. Berrange.

* configure.ac (--with-driver-modules): Fix help test.
* src/Makefile.am (libvirt_driver_xen.la, libvirt_driver_libxl.la)
(libvirt_driver_qemu.la, libvirt_driver_lxc.la)
(libvirt_driver_uml.la): Factor into new convenience libraries.
* tests/Makefile.am (xen_LDADDS, qemu_LDADDS, lxc_LDADDS)
(networkxml2argvtest_LDADD): Link to convenience libraries, not
shared libraries.