scsi: turn "is this a SCSI device?" into a conditional hint

If the user does not have permissions to send ioctls to the device (due to
SELinux or cgroups, for example), the output can look like

qemu-kvm: -device scsi-block,drive=disk: cannot get SG_IO version number:
  Operation not permitted.  Is this a SCSI device?

but this is confusing because the ioctl was blocked _before_ the device
even received the SG_GET_VERSION_NUM ioctl.  Therefore, for EPERM errors
the suggestion should be eliminated.  To make that simpler, change the
code to use error_append_hint.
Reported-by: NAla Hino <ahino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-disk.c

@@ -2607,9 +2607,10 @@ static void scsi_block_realize(SCSIDevice *dev, Error **errp)
     /* check we are using a driver managing SG_IO (version 3 and after) */
     rc = blk_ioctl(s->qdev.conf.blk, SG_GET_VERSION_NUM, &sg_version);
     if (rc < 0) {
-        error_setg(errp, "cannot get SG_IO version number: %s.  "
-                     "Is this a SCSI device?",
-                     strerror(-rc));
+        error_setg_errno(errp, -rc, "cannot get SG_IO version number");
+        if (rc != -EPERM) {
+            error_append_hint(errp, "Is this a SCSI device?\n");
+        }
         return;
     }
     if (sg_version < 30000) {

hw/scsi/scsi-generic.c

@@ -500,9 +500,10 @@ static void scsi_generic_realize(SCSIDevice *s, Error **errp)
     /* check we are using a driver managing SG_IO (version 3 and after */
     rc = blk_ioctl(s->conf.blk, SG_GET_VERSION_NUM, &sg_version);
     if (rc < 0) {
-        error_setg(errp, "cannot get SG_IO version number: %s.  "
-                         "Is this a SCSI device?",
-                         strerror(-rc));
+        error_setg_errno(errp, -rc, "cannot get SG_IO version number");
+        if (rc != -EPERM) {
+            error_append_hint(errp, "Is this a SCSI device?\n");
+        }
         return;
     }
     if (sg_version < 30000) {