Help QEMU in deprecation of -drive if=none without the need to refactor
all old boards. Stop masking out -blockdev support when -drive if=sd
needs to be used. We achieve this by forbidding blockjobs and
special-casing all other code paths. Blockjobs are sacrificed in this
case as SD cards are a corner case for some ARM boards and are thus not
used commonly.

https://bugzilla.redhat.com/show_bug.cgi?id=1821692Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>