hmp: Ignore Error objects where the return value suffices

hmp: Ignore Error objects where the return value suffices

qdev_print_props() receives and throws away Error objects just to
check for object_property_get_str() and object_property_print()
failure.  Unnecessary, both return suitable values, so use those
instead.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20200707160613[email protected]>