os.firmware="efi" silently clears firmware path

Software environment

• Operating system: linux
• Architecture: x86_64
• kernel version: all
• libvirt version: 8.4.0
• Hypervisor and version: qemu 7.0.0

Description of problem

when <os firmware='efi'> is set, the default firmware path overrides the path in the XML, erasing it.

Steps to reproduce

1. edit a domain xml
2. alter os entry to:

<os firmware='efi'>
    <type arch='x86_64' machine='pc-q35-7.0'>hvm</type>
    <loader>/opt/OVMF/OVMF_CODE.fd</loader>
    <boot dev='hd'>
</os>

3. Upon saving, loader path is removed.
4. after removing firmware='efi' attribute from os everything behaves as expected.

Additional information

The xml configuration passes checks, and other XML changes made at the same time are saved.

I suggest either throwing an error if os.firmware="efi" and the loader path is set, or only setting the default if the entry is empty.