Create biosboot partitions in blivet tests
    
    From anaconda-37.12.1, anaconda defaults to GPT for all BIOS
    installs. So we need to create a BIOS boot partition when doing
    a BIOS install. I think all other potential configs (x86_64
    UEFI, aarch64 (UEFI), ppc64le (OFW)) are covered under the other
    two paths, so just making this `else` should be OK.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>