schema: Remove xenmigr from host migrate transport capability

Commit id '1dac5fbb' removed xenmigr as a capability option
for virCapabilitiesAddHostMigrateTransport but didn't update
the schema resulting in possible failure for virt-xml-validate.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
ACKed-by Michal Privoznik <mprivozn@redhat.com>

docs/schemas/capability.rng

@@ -162,7 +162,6 @@
               <choice>
                 <value>esx</value>
                 <value>tcp</value>
-                <value>xenmigr</value>
               </choice>
             </element>
           </oneOrMore>