domaincaps: Expose UEFI binary path, if it exists

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>