launch: appliance: Add custom parameters last.

This allows custom parameters to modify parameters added by
libguestfs, eg. by doing:

  -set drive.hd0.file=rbd:foo/bar

[infernix adds the comment below:]

This works for recent versions of QEMU. The way to use this is:

guestfish
><fs> add-drive /dev/null
><fs> config -set drive.hd0.file=rbd:pool/volume
><fs> run

This will probably work for more types (sheepdog, iscsi et al), but
with the caveat that the libguestfs layers are bypassed so you'll
*always* end up directly accessing the disk in rw mode.