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>

tests/domaincapsschemadata/domaincaps-full.xml

@@ -6,6 +6,8 @@
   <vcpu max='255'/>
   <os supported='yes'>
     <loader supported='yes'>
+      <value>/foo/bar</value>
+      <value>/tmp/my_path</value>
       <enum name='type'>
         <value>rom</value>
         <value>pflash</value>

tests/domaincapsschemadata/domaincaps-qemu_1.6.50-1.xml

@@ -5,6 +5,7 @@
   <arch>x86_64</arch>
   <os supported='yes'>
     <loader supported='yes'>
+      <value>/usr/share/OVMF/OVMF_CODE.fd</value>
       <enum name='type'>
         <value>rom</value>
         <value>pflash</value>