qemuBlockStorageSourceDetachPrepare: Prepare for possibly missing 'format' layer
Setup the data for detaching of the 'format' layer only when it's
present.
Restructure the logic to follow the same order as
qemuBlockStorageSourceAttachPrepareBlockdev in terms of
format/slice/storage -blockdev objects, and drop the now-misleading
comment for 'slice' of raw disks.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>