Enable QEMU snapshots function for virtio-gpu
Previously the qemu version of qemu_x86_64 worker was too old and didn't support take snapshots with virtio-gpu.
So we disabled it by: https://github.com/os-autoinst/os-autoinst/pull/832
But the qemu version was updated to "QEMU emulator version 2.9.1(openSUSE Leap 42.3)" now, we should drop this line:
to enable the snapshots function for virtio-gpu.