conf: Format and parse private data for virDomainNetDef

conf: Format and parse private data for virDomainNetDef

The virDomainNetDef struct has privateData (which is currently
used by QEMU driver to store FDs opened during cmd line building
phase and pass them onto cmd line).

Soon, we will need to store additional information that needs to
survive daemon restart. Let's introduce machinery for parsing and
formatting privateData.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>