util: make virNetDev(Get|Set)IFFlags() static

util: make virNetDev(Get|Set)IFFlags() static

e562a61a0739 added these two new helper functions and only used them
within virnetdev.c, but declared them in the .h file. If some
currently unsupported interface flags need to be accessed in the
future, it will make more sense to write the appropriate higher level
function rather than require us to artificially define IFF_* on some
mythical platform that doesn't have SIOC[SG]IFFLAGS (and therefore
doesn't have IFF_*) just so we can call virNetDevSetIFFFlags() to
return an error.

To help someone in not going down the wrong road, this patch makes the
two helper functions static, hopefully making it less likely that
someone will want to use them outside of virnetdev.c.