schema: pool: netfs: Don't enforce slash in glusterfs pool source

Gluster volumes don't start with a leading slash. Our schema for netfs
gluster pools enforces it though. Luckily mount.glusterfs skips it.
Allow a slashless volume name for glusterfs netfs mounts in the schema.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1101999

tests/storagepoolxml2xmlin/pool-netfs-gluster-without-slash.xml

@@ -0,0 +1,12 @@
+<pool type='netfs'>
+  <source>
+    <host name='example.com'/>
+    <format type='glusterfs'/>
+    <dir path='volume'/>
+  </source>
+  <name>netfs-gluster</name>
+  <uuid>d5609ced-94b1-489e-b218-eff35c30336a</uuid>
+  <target>
+    <path>/mnt/gluster</path>
+  </target>
+</pool>