qdev: Don't leak string property value on hot unplug
parse_string() qemu_strdup()s the property value. It is never freed.
It needs to be freed along with the device. Otherwise, the value of
scsi-disk property "ver" gets leaked when hot-unplugging the disk, for
instance.
Call new PropertyInfo method free() from qdev_free(). Implement it
for qdev_prop_string.
Signed-off-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>