network: log error for unknown virNetworkUpdate command codes

network: log error for unknown virNetworkUpdate command codes

Every level of the code for virNetworkUpdate was assuming that some
other level was checking for validity of the "command" arg, but none
actually were. The result was that an invalid command code would do
nothing, but also report success.

Since the command code isn't used until the very lowest level backend
functions, that's where I put the check. I made a separate one-line
function to log the error. The compiler would have combined the
identical strings used by multiple calls if I'd just called
virReportError directly in each location, but sending them all to the
same string in the source guards against inadvertant divergence (which
would lead to extra work for translators.)