qdev_prop_set_string uses object_property_set_str, which takes
a const char * for the value. Lets propagate the constness
into qdev_prop_set_string.
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>