storage: fix RNG validation of gluster via netfs

storage: fix RNG validation of gluster via netfs

While trying to compare netfs against my new gluster pool, I
discovered two things:

virt-xml-validate chokes on valid xml produced by 'virsh pool-dumpxml'
[yet another reason that ALL patches that add new xml should be adding
corresponding tests]

When using glusterfs FUSE mounts, you cannot access a subdirectory
of a gluster volume.  The recommended workaround in the gluster
community is to mount the volume to an intermediate location, then
bind-mount the desired subdirectory to the final location.  Maybe
we should teach libvirt to do bind-mounting, but for now I chose to
just document the limitation.

* docs/storage.html.in: Improve documentation.
* docs/schemas/storagepool.rng (sourcefmtnetfs): Allow all
formats, and drop redundant info-vendor.
* tests/storagepoolxml2xmltest.c (mymain): New test.
* tests/storagepoolxml2xmlin/pool-netfs-gluster.xml: New file.
* tests/storagepoolxml2xmlout/pool-netfs-gluster.xml: Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>