ISCSI: Add support for thin-provisioning via discard/UNMAP and bigger LUNs

Update the configure test for libiscsi support to detect version 1.3
or later.  Version 1.3 of libiscsi provides both READCAPACITY16 as well
as UNMAP commands.

Update the iscsi block layer to use READCAPACITY16 to detect the size of
the LUN instead of READCAPACITY10. This allows support for LUNs larger
than 2TB.

Update to implement bdrv_aio_discard() using the UNMAP command.
This allows us to use thin-provisioned LUNs from TGTD and other iSCSI
targets that support thin-provisioning.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
[squashed in subsequent patch from Ronnie to fix off-by-one in LBA count]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
