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>
spapr-vscsi: Adding VSCSI capabilities

This implements capabilities exchange between vscsi host and client.  As
at the moment no capability is supported, put zero flags everywhere and
return.

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com>
spapr-vscsi: Report error on unsupported MAD requests

The existing driver just dropped unsupported requests. This adds error
responses to those unhandled requests.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>