From vJunos OS Release 24.1 release, you must boot vJunosEvolved only with UEFI BIOS. Otherwise, the BIOS hangs when you boot.
https://www.juniper.net/documentation/us/en/software/vJunosEvolved/vjunos-evolved-kvm/topics/vjunos-evolved-deploy-on-kvm.html#vJunosEvolved-kvm-set-host__note_bz4_jwv_mdc

FYI, dev...a-v-popov:netlab:ptx-uefi