Undefine NVRAM on vm destruction

VMs with UEFI store UEFI variables in OVMF files. This flag tells
Libvirt to undefine it, if it exists. This was introduced in libvirt
1.2.6, which can be found on very old EL7 systems, but those are no
longer supported. EL 7.9 (only supported EL7 version) ships libvirt 4.5.

lib/fog/libvirt/models/compute/server.rb

@@ -90,9 +90,9 @@ def disk_path
           volumes.first.path if volumes and volumes.first
         end
 
-        def destroy(options={ :destroy_volumes => false, :flags => 0 })
+        def destroy(options={ :destroy_volumes => false, :flags => ::Libvirt::Domain::UNDEFINE_NVRAM })
           poweroff unless stopped?
-          if options.fetch(:flags, 0).zero?
+          if options.fetch(:flags, ::Libvirt::Domain::UNDEFINE_NVRAM).zero?
             service.vm_action(uuid, :undefine)
           else
             service.vm_action(uuid, :undefine, options[:flags])