domaincaps: Expose UEFI binary path, if it exists

Check to see if the UEFI binary mentioned in qemu.conf actually
exists, and if so expose it in domcapabilities like

<loader ...>
  <value>/path/to/ovmf</value>
</loader>

We introduce some generic domcaps infrastructure for handling
a dynamic list of string values, it may be of use for future bits.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

docs/formatdomaincaps.html.in

@@ -105,6 +105,7 @@
   ...
   <os supported='yes'>
     <loader supported='yes'>
+      <value>/usr/share/OVMF/OVMF_CODE.fd</value>
       <enum name='type'>
         <value>rom</value>
         <value>pflash</value>
@@ -122,6 +123,11 @@
     <p>For the <code>loader</code> element, the following can occur:</p>
 
     <dl>
+      <dt>value</dt>
+      <dd>List of known loader paths. Currently this is only used
+      to advertise known locations of OVMF binaries for qemu. Binaries
+      will only be listed if they actually exist on disk.</dd>
+
       <dt>type</dt>
       <dd>Whether loader is a typical BIOS (<code>rom</code>) or
       an UEFI binary (<code>pflash</code>). This refers to