libvirt: Skip unsupported firmware types

Ignore (1) stateless mode firmware and (2) memory device firmware which
do not include a few core keys such as nvram-template. This is
a temporal (and backportable) workaround until firmware detection using
libvirt's internal feature is implemented by [1]

[1] https://blueprints.launchpad.net/nova/+spec/libvirt-firmware-auto-selection

Closes-Bug: #2122288
Change-Id: I99bc36fdd5df816c9ae374db71e4734fb7fc467b
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/bug-2133416-80522b523ee74835.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    [`bug 2133416 <https://bugs.launchpad.net/nova/+bug/2133416>`_] Libvirt
+    virt driver now ignores the OVMF firmware files of the following types.
+    Previously nova failed to start instances with UEFI boot when it attempts
+    to check such firmware files.
+
+    - stateless mode
+    - memory device