qdev: Improve diagnostics for bad property values

qdev: Improve diagnostics for bad property values

Property "vlan" reports "failed to parse" even when the value parses
just fine, but the result doesn't name an existing VLAN.

Similarly, properties "drive", "chr" and "netdev" misleadingly report
"failed to parse" when the value doesn't name an existing host device.

Change PropertyInfo method parse to return an error code, so that
qdev_prop_parse() can report the error more accurately.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>