cmdlib: Use -hda/-hdb instead of virtio-scsi incantations

[cgwalters]

Qemu has convenient shorthand CLI arguments to do what we
want here. We're explicitly not using virtio because supermin
hardcodes /dev/sda sadly.

Prep for changing this code to use kola qemuexec.

[jlebon]

/lgtm

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[Prashanth684]

unfortunately i think these scsi incantations are needed for s390x and ppc64le :(

supermin: waiting another 1024000000 ns for /sys/block/sda/dev to appear
This usually means your kernel doesn't support virtio, or supermin was unable
to load some kernel modules (see module loading messages above).
supermin: waiting another 2048000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 4096000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 8192000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 16384000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 32768000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 65536000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 131072000000 ns for /sys/block/sda/dev to appear
supermin: waiting another 262144000000 ns for /sys/block/sda/dev to appear
supermin: creating /dev/root as block special 18:0
supermin: mounting new root on /root
mount: /root: No such device or address
[  524.788014] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100
[  524.788080] CPU: 15 PID: 1 Comm: init Not tainted 5.5.11-200.fc31.ppc64le #1
[  524.788142] Call Trace:
[  524.788169] [c00000007d52bc40] [c000000000fbaf84] dump_stack+0xb8/0x104 (unreliable)
[  524.788230] [c00000007d52bc80] [c00000000016a5fc] panic+0x168/0x40c
[  524.788281] [c00000007d52bd10] [c000000000174090] do_exit+0xd50/0xdc0
[  524.788330] [c00000007d52bdd0] [c0000000001741cc] sys_exit_group+0x5c/0x110
[  524.788379] [c00000007d52be20] [c00000000000b278] system_call+0x5c/0x68
+ '[' '!' -f /srv/tmp/rc ']'
+ fatal 'Couldn'\''t find rc file, something went terribly wrong!'
+ echo 'fatal: Couldn'\''t find rc file, something went terribly wrong!'
fatal: Couldn't find rc file, something went terribly wrong!
+ exit 1

[cgwalters]

OK #1292 for now.