conf: Mark missing optional USB devices in domain XML

When startupPolicy set for a USB devices allows such device to be
missing, there was no way this could be detected from domain XML. With
this patch, libvirt emits a new missing='yes' attribute for such devices
when active domain XML is generated.

docs/schemas/domaincommon.rng

@@ -1654,6 +1654,14 @@
           </optional>
           <interleave>
             <element name="source">
+              <optional>
+                <attribute name="missing">
+                  <choice>
+                    <value>yes</value>
+                    <value>no</value>
+                  </choice>
+                </attribute>
+              </optional>
               <choice>
                 <group>
                   <ref name="usbproduct"/>

src/conf/domain_conf.c

@@ -12102,6 +12102,11 @@ virDomainHostdevSourceFormat(virBufferPtr buf,
     if (def->source.subsys.u.usb.autoAddress &&
         (flags & VIR_DOMAIN_XML_MIGRATABLE))
         virBufferAddLit(buf, " autoAddress='yes'");
+
+    if (def->missing &&
+        !(flags & VIR_DOMAIN_XML_INACTIVE))
+        virBufferAddLit(buf, " missing='yes'");
+
     virBufferAddLit(buf, ">\n");
 
     virBufferAdjustIndent(buf, 2);