Install the sysutils/vm-bhyve package.
add the following to /path/to/vm/.templates/slackware.conf
loader="uefi" cpu=4 graphics="yes" memory=8192M network0_type="virtio-net" network0_switch="public" disk0_type="virtio-blk" disk0_name="disk0.img"
vm create -t slackware -s 128G slackguest vm install slackguest slackware64-15.0-install-dvd.iso
partiton the disk cfdisk /dev/disk
run the setup
setup
and continue with the install..
Install ELILO since we setup Slackware to use uefi this is your only option and allows you to not require to use the bhyve specific version of grub.