qemu: command: Refactor disk commandline formatting

qemu: command: Refactor disk commandline formatting

Now that we have one place that sets up all disk-related objects to
qemuBlockStorageSourceAttachDataPtr we can easily reuse the data in the
command-line formatter by implementing a worker which will convert the
data.

A huge advantage is that it will be way easier to integrate this with
-blockdev later on.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>