VMTests: Fix legacy boot tests. (#346)

In newer versions of libvirt, specifying the `nvram` element in the
domain XML will result in the VM being interpreted as UEFI firmware
based. Whereas in older versions of libvirt, the `nvram` element is
ignored if the other UEFI elements (e.g. `loader`) are not specified.

This change drops the `nvram` element when legacy boot is specified.
This fixes the `libvirt: Domain Config error : no loader path specified
and firmware auto selection disabled` error.

Author: cwize1

test/vmtests/vmtests/utils/libvirt_utils.py

@@ -160,7 +160,8 @@ def create_libvirt_domain_xml(libvirt_conn: libvirt.virConnect, vm_spec: VmSpec)
     os_type.attrib["arch"] = host_arch
     os_type.attrib["machine"] = machine_model
 
-    ET.SubElement(os_tag, "nvram")
+    if vm_spec.boot_type == "efi":
+        ET.SubElement(os_tag, "nvram")
 
     os_boot = ET.SubElement(os_tag, "boot")