block/iscsi: bump libiscsi requirement to 1.9.0

This patch lifts the minimum supported libiscsi version from 1.4.0 to
1.9.0 since the BUSY patch required that change.

On one this allows us to remove all #ifdefs from the code which
makes the code easier to maintain and read. On the other hand
I would not recommend libiscsi prior to 1.8.0 for production use
because the following important libiscsi fixes for deadlocks and
protocol errors are missing prior to 1.8.0:

dbe9a1e SOCKET queue cmd PDUs directly in waitpdu queue
30df192 DATA-OUT set pdu->cmdsn appropriately
548bd22 ISCSI fix broken send logic in iscsi_scsi_async_command
14bee10 RECONNECT do not increase CmdSN for immediate PDUs
1f4a66a PDU queue out PDUs in order of itt.
562dd46 PDU avoid incrementing itt to 0xffffffff
cd09c0f PDU use serial32 arithmetic for cmdsn, maxcmdsn and expcmdsn.
89e918e SOCKET validate data_size in in_pdu header
91267f5 Limit immediate and unsolicited data to FirstBurstLength

Note that libiscsi 1.9.0 was released on Feb 24th, 2013, about
one month after 1.8.0.

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