iscsi: add support for iSCSI NOPs [v2]

iscsi: add support for iSCSI NOPs [v2]

This patch will send NOP-Out PDUs every 5 seconds to the iSCSI target.
If a consecutive number of NOP-In replies fail a reconnect is initiated.
iSCSI NOPs help to ensure that the connection to the target is still operational.
This should not, but in reality may be the case even if the TCP connection is still
alive if there are bugs in either the target or the initiator implementation.

v2:
 - track the NOPs inside libiscsi so libiscsi can reset the counter
   in case it initiates a reconnect.

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