Allow a base label to be specified in dynamic labelling mode

Normally the dynamic labelling mode will always use a base
label of 'svirt_t' for VMs. Introduce a <baselabel> field
in the <seclabel> XML to allow this base label to be changed

eg

   <seclabel type='dynamic' model='selinux'>
     <baselabel>system_u:object_r:virt_t:s0</baselabel>
   </seclabel>

* docs/schemas/domain.rng: Add <baselabel>
* src/conf/domain_conf.c, src/conf/domain_conf.h: Parsing
  of base label
* src/qemu/qemu_process.c: Don't reset 'model' attribute if
  a base label is specified
* src/security/security_apparmor.c: Refuse to support base label
* src/security/security_selinux.c: Use 'baselabel' when generating
  label, if available

docs/schemas/domain.rng

@@ -67,6 +67,9 @@
       <element name="imagelabel">
         <text/>
       </element>
+      <element name="baselabel">
+        <text/>
+      </element>
     </element>
   </define>
   <define name="hvs">