Add support for external DNS/Static IPs, as well as bridged adapters other than virbr0

Many networks have external IPAM and/or DHCP servers to hand out IP addresses for VMs.

It would be great if a non-libvirt managed DHCP server could be used with Nomad's Virt driver.

Ideally, Nomad could use the builtin capability for qemu-agent to execute and return information from the guest's network adapters.

This method would enable almost universal support for different network configurations, like static IPs retrieved from an IPAM solution like Netbox/Infoblox, and enable bridged adapters other than the libvirt managed `virbr0`.

It would require qemu-guest-agent package to be installed inside the guest VM.

Example:
`virsh qemu-agent-command virt-task-4930bb25 '{"execute":"guest-network-get-interfaces"}' | jq .`

This returns JSON which could be parsed by Nomad:
```
{
"return": [
{
"name": "lo",
"ip-addresses": [
{
"ip-address-type": "ipv4",
"ip-address": "127.0.0.1",
"prefix": 8
},
{
"ip-address-type": "ipv6",
"ip-address": "::1",
"prefix": 128
}
],
"statistics": {
"tx-packets": 1875360,
"tx-errs": 0,
"rx-bytes": 133160856,
"rx-dropped": 0,
"rx-packets": 1875360,
"rx-errs": 0,
"tx-bytes": 133160856,
"tx-dropped": 0
},
"hardware-address": "00:00:00:00:00:00"
},
{
"name": "enp0s5",
"ip-addresses": [
{
"ip-address-type": "ipv4",
"ip-address": "10.14.30.29",
"prefix": 24
},
{
"ip-address-type": "ipv6",
"ip-address": "redacted",
"prefix": 64
}
],
"statistics": {
"tx-packets": 958693,
"tx-errs": 0,
"rx-bytes": 439869753,
"rx-dropped": 176893,
"rx-packets": 1563633,
"rx-errs": 0,
"tx-bytes": 80836052,
"tx-dropped": 0
},
"hardware-address": "redacted"
}
]
}
```

Start by tracing the Virt driver's existing libvirt networking path, especially its reliance on virbr0, and review the qemu-agent guest-network-get-interfaces command shown in the issue. Define how external DHCP or IPAM, static addresses, and other bridged adapters should be represented and verified; done means these network configurations work through the driver.

Written by the indexing model from the issue text.