ui: guest import: rework windows virtio-scsi preparation

ui: guest import: rework windows virtio-scsi preparation

instead of having a separate iso selector that shows with an enabled
checkbox, just add a CDROM drive when windows is selected and there is
no such drive available yet.
The idea here is that the VM's OS is already fully set up, so a single
CDROM drive is enough to be used for installing VirtIO drivers, unlike
the VM create case, where the first one is already used for the
installation medium.

Also, rename the 'map to sata' checkbox to 'prepare for virtio-scsi'
that also changes the scsi controller to virtio-scsi-single

Additionally, change the positioning of the checkbox/scsihw selector
to be below the disk grid
With that we then only disable prepare-for-virtio checkbox for
non-windows OS types, as the scsi controller on the right looks like
it hangs in the air without any field on the left otherwise.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
 [ TL: rework commit message, squash in some fixes ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>