vl: Create block backends before setting machine properties

qemu-system-FOO's main() acts on command line arguments in its own
idiosyncratic order.  There's not much method to its madness.
Whenever we find a case where one kind of command line argument needs
to refer to something created for another kind later, we rejigger the
order.

Block devices get created long after machine properties get processed.
Therefore, block device machine properties can be created, but not
set.  No such properties exist.  But the next commit will create some.
Time to rejigger again: create block devices earlier.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190308131445.17502-8-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>