vm/qemu: fix default flags for arm/arm64
vm/qemu: fix default flags for arm/arm64

The console and root that we use are specific for 386/amd64.
arm/arm64 usually use a different console and root device.
Use separate settings for 386/amd64 and arm/arm64.

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