iscsi: split discard requests in multiple parts

Replace .bdrv_aio_discard with .bdrv_co_discard so that discard
requests can be split in multiple parts, each for a small amount
of sectors.

This is useful because we expose a generic API with no limit
on the amount of sectors that can be unmapped in one request.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
