We use typedef IMedium IHardDisk to make IHardDisk hierachy from
IMedium (Actually it did on vbox 2.2 and 3.0's C++ API).
So when calling
    VBOX_MEDIUM_FUNC_ARG*(IHardDisk, func, args)
we can directly replace it to
    gVBoxAPI.UIMedium.func(IHardDisk, args)

When dealing with this two types, we get some rules from it's
hierachy relationship.

When using IHardDisk and IMedium as input, we can't transfer a
IMedium to IHardDisk. Like:
    gVBoxAPI.UIHardDisk.func(IHardDisk *hardDisk, args)
    Here, we can't put a *IMedium as a argument.

When using IHardDisk and IMedium as output, we can't transfer a
IHardDisk to IMedium. Like:
    gVBoxAPI.UIMachine.GetMedium(IMedium **out)
    Here, we can't put a **IHardDisk as a argument. If this case
    do happen, we either change the API to GetHardDisk or write a
    new one.

This patch rewrites the following functions
    *vboxStorageOpen
    *vboxStorageClose
    *vboxConnectNumOfStoragePools
    *vboxConnectListStoragePools
    *vboxStoragePoolLookupByName

These functions do not call any vbox API, so I directly move it
from vbox_tmpl.c to vbox_storage.c

A small improvement is made on vboxConnectListStoragePools.
The if condition nnames == 1 is modified to nnames > 0. So if the
caller put more than one slot to get active storage pools, the new
function will return exactly one, while the old one would only
return 0.

I made a mistake on copyright in patch 7f0f415b.

If I copied codes from one file to another, I should copy the
copyright announcement as well. So this patch makes up the
copyright which I should have added in the previous patch.

To prepare for introducing a single global driver, rename the
virDriver struct to virHypervisorDriver and the registration
API to virRegisterHypervisorDriver()

This patch seperate the domain driver and the network driver.

libvirt_driver_vbox_impl.la has been linked in the network driver.
So that the version specified codes in vbox_V*.c would only be
compiled once.

The vboxGetNetworkDriver provides a simple interface to get vbox
network driver.

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.