Our hotplug test cases are weak in comparison to the qemuxml2argvtest.
Use all the the input data to also validate the arguments for -netdev
and -blockdev against the appropriate commands of the QMP schema.

Note that currently it's done just for the _CAPS versions of tests but
commenting out a line in the test file allows to validate even cases
which don't use real capabilities.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>