Preserve UEFI NVRAM variable store

Preserve NVRAM variable store during stop/start, hard reboot, live
migration, and volume retype.

This does not affect cold migration or shelve.

For UEFI guests (hw_firmware_type=uefi), every time the instance is
started, the UEFI variable storage for that instance
(/var/lib/libvirt/qemu/nvram/instance-xxxxxxxx_VARS.fd) is deleted
and reinitialized from the default template.

The changes are based on this patch by Jonas Schäfer to preserve the
vTPM state:
https://review.opendev.org/c/openstack/nova/+/955657

Closes-Bug: #1633447
Closes-Bug: #2131730
Change-Id: I444a9285c07a04bf08a73772235f8dd73d75e513
Signed-off-by: Nicolai Ruckel <nicolai.ruckel@cloudandheat.com>

releasenotes/notes/preserve-nvram-ab6d3d2fe923301f.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    NVRAM variable store is preserved during stop/start, hard reboot, and live
+    migration by passing the corresponding flag to libvirt.
+
+    See https://bugs.launchpad.net/nova/+bug/1633447 for more details.