domaincapstest: add bhyve caps test

domaincapstest: add bhyve caps test

 * 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