SD cards need to be instantiated via -drive if=sd. This means that all
cases where we use the blockdev path need to be special-cased for SD
cards.

Note that at this point QEMU_CAPS_BLOCKDEV is still cleared if the VM
config has a SD card.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>