qemu_capabilities: Put only unique FW images into domcaps

qemu_capabilities: Put only unique FW images into domcaps

In the domain capabilities XML there are FW image paths printed.
There are two sources for the image paths (in order of
preference):

  1) firmware descriptor files - as returned by
  qemuFirmwareGetSupported()

  2) a compile time list of FW:NRAM pairs which can be overridden
  in qemu.conf

If either of those contains a duplicate FW image path (which is
a valid use case) it is printed twice in the capabilities XML.
While it's technically not a bug, it doesn't look good.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>