Add libvirt-admin library

Add libvirt-admin library

Initial scratch of the admin library.  It has its own virAdmConnectPtr
that inherits from virAbstractConnectPtr and thus trivially supports
error reporting.

There's pkg-config file added and spec-file adjusted as well.

Since the library should be "minimalistic" and not depend on any other
library, the list of files is especially crafted for it.  Most of them
could've been put to it's own sub-libraries that would be LIBADD'd to
libvirt_util, libvirt_net_rpc and libvirt_setuid_rpc_client to minimize
the number of object files being built, but that's a refactoring that
isn't the orginal aim of this commit.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>