* Extract filling bhyve capabilities from virBhyveDomainCapsBuild()
into a new function virBhyveDomainCapsFill() to make testing
easier by not having to mock firmware directory listing and
hypervisor capabilities probing
* Also, just presence of the firmware files is not sufficient
to enable os.loader.supported, hypervisor should support UEFI
boot too
* Add tests to domaincapstest for the main caps possible flows:
- when UEFI bootrom is supported
- when video (fbus) is supported
- neither of above is supported
