Boxes should create machines using UEFI by default (OVMF)
libvirt supports OVMF (QEMU's implementation of UEFI) for a few years now: https://vfio.blogspot.com.es/2014/09/libvirt-now-supports-ovmf.html
Most modern desktop machines (10+ years) don't use legacy BIOS boot anymore and most modern OSes are targeting the UEFI bootpath and features.