vm/vmm: make more robust after restarts
vm/vmm: make more robust after restarts

Instances from the previous run can still be running,
so always stop the previous instances before starting new ones.

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