r->buf is hardcoded to 2056 which is (256 + 1) * 8, allowing 256 luns at
most. If more than 256 luns are specified by user, we have buffer
overflow in scsi_target_emulate_report_luns.

To fix, we allocate the buffer dynamically.
Signed-off-by: NAsias He <asias@redhat.com>
Tested-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>