This patch rewrites two public APIs. They are vboxNetworkUndefine
and vboxNetworkDestroy. They use the same core function
vboxNetworkUndefineDestroy. I merged it in one patch.

This patch actually contains two public API, virNetworkDefineXML
and virNetworkCreateXML. They use the same core function
vboxNetworkDefineCreateXML. So I merged it together.

After this patch, we will have only one common domain driver.
So, the rewritten on domain driver is finished in this patch.

Four functions are rewrite in this patch, that is:
vboxNodeGetInfo
vboxNodeGetCellsFreeMemory
vboxNodeGetFreeMemory
vboxNodeGetFreePages
Since these functions has nothing to do with vbox,
it can be directly moved to vbox_common.c. So, I
merged these things into one patch.

The vboxDomainSnapshotCreateXML integrated the snapshot redefine
with this patch:
http://www.redhat.com/archives/libvir-list/2014-May/msg00589.html
This patch introduced vboxSnapshotRedefine in vboxUniformedAPI to
enable the features.

This patch replace all version specified APIs to the uniformed api,
then, moving the whole implementation to vbox_common.c. As there
is only API level changes, the behavior of the function doesn't
change.

Some old version's defects has brought to the new one. The already
known things are:
    *goto cleanup in a loop without releasing the pointers in the
    loop.
    *When function failed after machine unregister, no roll back
    to recovery it and the virtual machine would disappear.

Since vboxDomainAttachDeviceImple not used in vbox_tmpl.c, it has
been deleted.