vbox: Rewrite vbox-independent functions
    
    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.