The API on IHardDiskAttachment is merged into IMediumAttachment.
So, we don't need it anymore.

If the <path> in target element is not given, libvirt will put the
new volume in ~/.VirtualBox by default.

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.