vm/vmm: use derived disks for VMs
vm/vmm: use derived disks for VMs

As a result, the boot time is significantly improved since there's no longer any
need to copy the complete disk.

This feature was recently committed to OpenBSD-current. Any existing base image
used must be recreated, this time using the qcow2 disk format.

google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller