libvirt: Use firmware auto-selection by libvirt

Use the firmware auto-selection feature in libvirt to find the best
UEFI firmware file according to the requested feature.

Firmware files may be reselected when a libvirt domain is created from
scratch, while these are kept during hard-reboot (or live migration
which preserves the loader/nvram elements filled by libvirt).

Closes-Bug: #2122296
Related-Bug: #2122288
Implements: blueprint libvirt-firmware-auto-selection
Change-Id: Ie48b020597a1a2fb3280815eec5ba3565e396f9b
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/bp-libvirt-firmware-auto-selection-2d58b075816898b7.yaml

@@ -0,0 +1,16 @@
+---
+features:
+  - |
+    Now the libvirt virt driver uses firmware auto-selection by libvirt, which
+    is capable to select the appropriate firmware files according to
+    the requested features. This built-in auto-selection extends the existing
+    firmware selection capability within nova, and checks a few more feature
+    flags such as amd-sev and also supports new firmware types such as rom
+    firmware.
+
+upgrades:
+  - |
+    Existing UEFI guests may start using a new firmware file after operations
+    like rebuild or resize (including cold-migration) which generates libvirt
+    domain XML from scratch, due to the extended logic to select
+    the appropriate files.