sriov: fix ipxe rom file issue

[meinaLi]

Relying on ipxe build in tests when cannot obtain VF pxe rom file is an unstable testing method. Because this case is mainly to test the xml but not the pxe function. So we decided to use the fixed virtio rom file to test.
The error message:
This test needs rom file: /usr/share/ipxe/8086154c.rom.

[meinaLi]

# avocado run --vt-type libvirt --vt-omit-data-loss --vt-machine-type q35 sriov.plug_unplug.attach_detach_device_special_situations.hostdev_interface.boot_order
JOB ID     : efbf93c082fc02489922db31020f522e0ba92df0
JOB LOG    : /var/log/avocado/job-results/job-2024-12-19T21.52-efbf93c/job.log
 (1/1) type_specific.io-github-autotest-libvirt.sriov.plug_unplug.attach_detach_device_special_situations.hostdev_interface.boot_order: STARTED
 (1/1) type_specific.io-github-autotest-libvirt.sriov.plug_unplug.attach_detach_device_special_situations.hostdev_interface.boot_order: PASS (71.52 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/log/avocado/job-results/job-2024-12-19T21.52-efbf93c/results.html
JOB TIME   : 73.61 s

[Yingshun]

I think you can use a more direct variable name instead of commenting here.

Suggested change

	# Directly use virtio rom file because it mainly test xml but not pxe function.
	rom_file = "/usr/share/ipxe/1af41000.rom"
	virtio_rom_file = "/usr/share/ipxe/1af41000.rom"
	if not os.path.exists(virtio_rom_file):
	self.test.error(f"This test needs a rom file, but neither {virtio_rom_file} or {rom_file} exist!")