JSON args for -netdev were added as precursor for adding the 'dgram'
network backend type. Enable the detection and update test cases using
DO_TEST_CAPS_LATEST.
Enabling the capability also ensures that the -netdev argument is
validated against the QAPI schema of 'netdev_add' which was already
implemented but not enabled.
The parser supporting JSON was added by qemu commit f3eedcddba3 and
enabled when adding stream/dgram netdevs in commit 5166fe0ae46.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
