block/iscsi: Drop iscsi_co_get_block_status for older versions of libiscsi

Debian wheezy includes libiscsi-dev 1.4.0 which does not provide
SCSI_PROVISIONING_TYPE_DEALLOCATED. Drop iscsi_co_get_block_status
in this case to allow compilation without errors.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>