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>
