admin: Introduce libvirt-admin.conf

Since libvirt-admin is a separate library, it should also have a separate
config file. Available settings are currently the same as for libvirt.conf.

src/libvirt-admin.conf

+#
+# This can be used to setup URI aliases for frequently
+# used connection URIs. Aliases may contain only the
+# characters  a-Z, 0-9, _, -.
+#
+# Following the '=' may be any valid libvirt admin connection
+# URI, including arbitrary parameters
+
+#uri_aliases = [
+#  "admin=libvirtd:///system",
+#]
+
+# This specifies the default location the client tries to connect to if no other
+# URI is provided by the application
+
+#uri_default = "libvirtd:///system"