scsi: fix allocation for s390x loadparm

scsi: fix allocation for s390x loadparm

Coverity reports a possible buffer overrun due to a non-NUL-terminated
string in scsi_property_set_loadparm().  While things are not so easy,
because qdev_prop_sanitize_s390x_loadparm is designed to operate on a
buffer that is not NUL-terminated, in this case the string *does* have
to be NUL-terminated because it is read by scsi_property_get_loadparm
and s390_build_iplb.

Reviewed-by: jrossi@linux.ibm.com
Cc: thuth@redhat.com
Fixes: 429442e52d9 ("hw: Add "loadparm" property to scsi disk devices for booting on s390x", 2024-11-18)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>