Enable tuning of qemu network tap device "sndbuf" size

Enable tuning of qemu network tap device "sndbuf" size

This is in response to a request in:

   https://bugzilla.redhat.com/show_bug.cgi?id=665293

In short, under heavy load, it's possible for qemu's networking to
lock up due to the tap device's default 1MB sndbuf being
inadequate. adding "sndbuf=0" to the qemu commandline -netdevice
option will alleviate this problem (sndbuf=0 actually sets it to
0xffffffff).

Because we must be able to explicitly specify "0" as a value, the
standard practice of "0 means not specified" won't work here. Instead,
virDomainNetDef also has a sndbuf_specified, which defaults to 0, but
is set to 1 if some value was given.

The sndbuf value is put inside a <tune> element of each <interface> in
the domain. The intent is that further tunable settings will also be
placed inside this element.

     <interface type='network'>
       ...
       <tune>
         <sndbuf>0</sndbuf>
       ...
       </tune>
     </interface>