blockjob: Fix error message for negative speed

blockjob: Fix error message for negative speed

The error message for a negative speed uses QERR_INVALID_PARAMETER,
which implies that the 'speed' option doesn't even exist:

    {"error": {"class": "GenericError", "desc": "Invalid parameter 'speed'"}}

Make it use QERR_INVALID_PARAMETER_VALUE instead:

    {"error": {"class": "GenericError", "desc": "Parameter 'speed' expects a non-negative value"}}

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>