virtio: use generic name when possible

We have two different virtio buses: pci and s390. The abstraction path
taken in qemu is to have generic aliases for each device type in the
architecture specific qdev devices.

So let's make use of these aliases whenever we can and define them
whenever we can.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>