virtio: block: dynamic maximum segments

Enhance the driver to handle whatever maximum segment number the host
tells us to handle.  Do to this, we need to allocate the scatterlist
dynamically.

We set max_phys_segments and max_hw_segments to the same value (1 if
the host doesn't tell us, since that's safest and all known hosts do
tell us).

Note that kmalloc'ing the structure for large sg_elems might be
problematic: the fix for this is sg_table, but that requires more
work.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>