error: Don't abuse qemu_error() for non-error in scsi_hot_add()

error: Don't abuse qemu_error() for non-error in scsi_hot_add()

Commit 30d335d6 converted an informational message from
monitor_printf() to qemu_error(), probably because the latter doesn't
need a mon argument.  A later commit will make qemu_error() print
additional stuff that is only appropriate for proper errors, and then
this will break.  Clean it up.