admin: Check for flags properly

admin: Check for flags properly

Function virAdmConnectListServers() forgot to check for flags at all,
virAdmConnectOpen() on the other hand checked them but did no dispatch
the error.  virCheckFlags() should be used only when there should be no
other thing done after erroring out and since they are used on different
places then just public API, they cannot dispatch errors.  So let's use
virCheckFlagsGoto instead.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>