schema: Add vzmigr for host migrate transport capability

Commit id '0eced74f' added vzmigr as a valid option for
virCapabilitiesAddHostMigrateTransport, but didn't update
the capabilities schema resulting in possible virt-xml-validate
failure.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
ACKed-by Michal Privoznik <mprivozn@redhat.com>

docs/schemas/capability.rng

@@ -163,6 +163,7 @@
                 <value>vpxmigr</value>
                 <value>tcp</value>
                 <value>rdma</value>
+                <value>vzmigr</value>
               </choice>
             </element>
           </oneOrMore>