missing Close() method for VirtualNetwork

Currently, there's no way to free the memory allocated by VirtualNetwork.

For instance, the following code will leak around ~100MB of memory:

```go
config := types.Configuration{
Debug: false,
CaptureFile: "",
MTU: 1500,
Subnet: "10.10.10.0/16",
GatewayIP: "10.10.10.1",
GatewayMacAddress: "12:34:56:78:aa:bb",
DHCPStaticLeases: map[string]string{},
DNS: []types.Zone{},
Forwards: map[string]string{},
NAT: map[string]string{},
GatewayVirtualIPs: []string{"10.10.10.254"},
Protocol: types.QemuProtocol,
}

fmt.Println("Memory usage before creating virtual networks:")
memStats()

for i := 0; i < 1000; i++ {
_, err := virtualnetwork.New(&config)
if err != nil {
panic(err)
}
}

runtime.GC()

fmt.Println("Memory usage after creating virtual networks:")
memStats()
```

Output:
```
Memory usage before creating virtual networks:
Alloc = 0 MiB TotalAlloc = 0 MiB Sys = 10 MiB NumGC = 0
Memory usage after creating virtual networks:
Alloc = 97 MiB TotalAlloc = 125 MiB Sys = 410 MiB NumGC = 6
```

The implementation of memStats() (taken from [here](https://gist.github.com/j33ty/79e8b736141be19687f565ea4c6f4226)):

```golang
func memStats() {
var m runtime.MemStats
runtime.ReadMemStats(&m)
// For info on each, see: https://golang.org/pkg/runtime/#MemStats
fmt.Printf("Alloc = %v MiB", bToMb(m.Alloc))
fmt.Printf("\tTotalAlloc = %v MiB", bToMb(m.TotalAlloc))
fmt.Printf("\tSys = %v MiB", bToMb(m.Sys))
fmt.Printf("\tNumGC = %v\n", m.NumGC)

}

func bToMb(b uint64) uint64 {
return b / 1024 / 1024
}
```