jsonrpc: cannot put link down on vNIC

There was no updateDevice method on VM object in the API module. The
name was vmUpdateDevice which is in conflict with current schema.

This method was missing from command_info so it was updated.

Change-Id: I81bd4fccdbb64594ed4592fed6b84b6fb07b357b
Bug-Url: https://bugzilla.redhat.com/1156399
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/34450
Reviewed-by: Dan Kenigsberg <danken@redhat.com>