virt-install/virt-manager have had this check forever:

def is_blktap_capable(conn):
    # Ideally we would get this from libvirt capabilities XML
    if conn.is_remote():
        return False

    f = open("/proc/modules")
    lines = f.readlines()
    f.close()
    for line in lines:
        if line.startswith("blktap ") or line.startswith("xenblktap "):
            return True
    return False

If True, we use driver name='tap' for <disk> XML. As the comment above suggestions, libvirt should expose some tag in capabilities so we know a
host supports blktap, and tools don't need to probe lsmod or similar.