syz-verifier: support multiple VMs (#3040)
syz-verifier: support multiple VMs (#3040)

Reused config parameter "config.vm.count".
Local fuzzing speed test result:
1 vm/pool => 380 programs/minute
2 vm/pool => 1050 programs/minute
4 vm/pool => 1600 programs/minute
8 vm/pool => 800 programs/minute
xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller