While trying to test all possible qemu machine on LAVA, I found that
most cannot ran tests since they are stored on a qcow2 image.
This qcow2 image is then connected by any of IDE, SCSI, virtio
via this qemu option
"-drive format=qcow2,file=/var/lib/lava/dispatcher/tmp/jobid/apply-overlay-guest-49d_bpy5/lava-guest.qcow2,media=disk,if=XXXX"

But for some QEMU machine all thoses bus are not availlable.
For example the qemu ARM cubieboard does not have any of thoses.
After some research and discussion on #qemu, the only way to add the LAVA qemu
test storage on cubieboard is with:
"-device ide-hd,drive=lavatest"
where drive=id must be the same as the id present on "-drive"

So this patch adds such id on the "-drive" option.
This id could be tuned via a new "guestfs_driverid" option.