xen-blkfront: Introduce a 'max' module parameter to alter the amount of indirect segments.

The max module parameter (by default 32) is the maximum number of
segments that the frontend will negotiate with the backend for indirect
descriptors.  Higher value means more potential throughput but more
memory usage. The backend picks the minimum of the frontend and its
default backend value.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>