Create a new "Prepare" function and move the drive add code into the new
helpers. This will eventually allow to simplify and unify the attaching
code for use with blockdev at the same time as providing compatibility
with older qemus.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>