qemu: use disk virtio by default

[mvo5]

As discussed in https://github.com/snapcore/spread/pull/96#pullrequestreview-367801844 the qemu backend should use virtio by default all the time.

This will be useful for UC20 where the UEFI grub bootloader is very very slow (minutes) when loading the kernel in non-virtio mode.

It's also a good idea in general as it will speedup the tests. The downside is that projects that hardcode /dev/sda will have to do a one time change to move to /dev/vda