qemu: eliminate memory leaks when converting NetDefs to type='ethernet'

qemu: eliminate memory leaks when converting NetDefs to type='ethernet'

in qemuConnectDomainXMLToNative. This function was only accounting for
about 1/10 of all the allocated items in the NetDef prior to memseting
it to all 0's. On top of that, it was going to great pains to learn
the name of the bridge device, but then never doing anything useful
with it (just putting it into data.ethernet.dev, which is *never* used
when building a qemu commandline). (I think this again all started off
as code with good intentions, but it was never completed, and instead
was just Frankensteinically cargo-culted into the odd mish mash we
have today).

The resulting code is much simpler, produces exactly the same output,
and doesn't leak memory.