Set managed to true for Terraform created networks

[joncave]

Using XenServer 7.4 I receive the following error when starting a VM with a newly Terraform created network:

BRIDGE_NOT_AVAILABLE

XenCenter states: "Could not find bridge required by VM".

Setting Managed to true in the NetworkRecord fixes it for me.

Reproduce with:

resource "xenserver_network" "test" {
  name_label = "test"
  description = "Test network"
  bridge = ""
}

Then attach this new network to a VM. It will no longer start.

[ringods]

Hello @joncave, could you rebase this PR from master plz? It conflicts with one of your previous PRs which is already merged.

[joncave]

@ringods Done.