storage: pool: Allow more intricate nfs protocol versions

Treat the 'protocolVer' field as a string so that e.g. '4.1' can be
used. Forbid only ',' in the string as it's a separator of arguments for
mount options.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/storagepoolxml2argvdata/pool-netfs-protocol-ver-linux.argv

@@ -1,5 +1,5 @@
 mount \
--o nodev,nosuid,noexec,nfsvers=3 \
+-o nodev,nosuid,noexec,nfsvers=4.1 \
 -t nfs \
 localhost:/var/lib/libvirt/images \
 /mnt