qemu: eliminate use of virDomainNetDefClear() in qemuConnectDomainXMLToNative()
qemu: eliminate use of virDomainNetDefClear() in qemuConnectDomainXMLToNative()

Instead of saving the interesting pieces of each existing NetDef,
clearing it, and then copying back the saved pieces after setting the
type to ethernet, just create a new NetDef, copy in the interesting
bits, and replace the old one. (The end game is to eliminate
virDomainNetDefClear() completely, since this is the only real use)

Signed-off-by: Laine Stump <c23361c43fbf79fed83e8b76173707b083d6caf5@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,jardasgit/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,jardasgit/libvirt,olafhering/libvirt,jfehlig/libvirt