xen-blkfront: rename indirect descriptor parameter

"max" is rather ambiguous and carries pretty little meaning, the more
that there are also "max_queues" and "max_ring_page_order". Make this
"max_indirect_segments" instead, and at once change the type from int
to uint (to match the respective variable's type).

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>