fix: unmarshal number leads to int overflow

[troyanov]

Use int64 type for disk size.

Resolves https://github.com/maas/terraform-provider-maas/issues/116

When compiled on a 32-bit platform int will be int32 and will lead to an overflow and JSON unmarshal error:

type Foo struct {
    Size int32
}

func main() {
    data := []byte(`{"Size": 128032178176}`)
    foo := Foo{}
    err := json.Unmarshal(data, &foo)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(foo.Size)
}

[troyanov]

Changes look good. Since you opened this one, what about memory (int) and storage, which is float64?!?

Same for VMHostResource

Well, my intent was to fix one particular issue and not all the type issues in the codebase, but if we want to address them all at once, I am fine with it 🙂

[skatsaounis]

Thank you very much for taking care of all the potential overflows. LGTM :beers: