conf: domcaps: Add 'backingStoreInput' domain capability

Historically we've only supported the <backingStore> as an output-only
element for domain disks. The documentation states that it may become
supported on input. To allow management apps detectin once that happens
add a domain capability which will be asserted if the hypervisor driver
will be able to obey the <backingStore> as configured on input.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/formatdomaincaps.html.in

@@ -516,6 +516,7 @@
     </gic>
     <vmcoreinfo supported='yes'/>
     <genid supported='yes'/>
+    <backingStoreInput supported='yes'/>
     <sev>
       <cbitpos>47</cbitpos>
       <reduced-phys-bits>1</reduced-phys-bits>
@@ -552,6 +553,13 @@
 
     <p>Reports whether the genid feature can be used by the domain.</p>
 
+    <h4><a id="featureBackingStoreInput">backingStoreInput</a></h4>
+
+    <p>Reports whether the hypervisor will obey the &lt;backingStore&gt;
+    elements configured for a &lt;disk&gt; when booting the guest, hotplugging
+    the disk to a running guest, or similar.
+    </p>
+
     <h4><a id="elementsSEV">SEV capabilities</a></h4>
 
     <p>AMD Secure Encrypted Virtualization (SEV) capabilities are exposed under

docs/schemas/domaincaps.rng

@@ -207,6 +207,9 @@
       <optional>
         <ref name='vmgenid'/>
       </optional>
+      <optional>
+        <ref name='backingStoreInput'/>
+      </optional>
       <optional>
         <ref name='sev'/>
       </optional>
@@ -232,6 +235,12 @@
     </element>
   </define>
 
+  <define name='backingStoreInput'>
+    <element name='backingStoreInput'>
+      <ref name='supported'/>
+    </element>
+  </define>
+
   <define name='sev'>
     <element name='sev'>
       <ref name='supported'/>

src/conf/domain_capabilities.c

@@ -40,6 +40,7 @@ VIR_ENUM_IMPL(virDomainCapsFeature,
               "iothreads",
               "vmcoreinfo",
               "genid",
+              "backingStoreInput",
 );
 
 static virClassPtr virDomainCapsClass;

src/conf/domain_capabilities.h

@@ -162,6 +162,7 @@ typedef enum {
     VIR_DOMAIN_CAPS_FEATURE_IOTHREADS = 0,
     VIR_DOMAIN_CAPS_FEATURE_VMCOREINFO,
     VIR_DOMAIN_CAPS_FEATURE_GENID,
+    VIR_DOMAIN_CAPS_FEATURE_BACKING_STORE_INPUT,
 
     VIR_DOMAIN_CAPS_FEATURE_LAST
 } virDomainCapsFeature;