deployment fails when install_kvm/register_vmhost is true

VM host registration fails with the traceback below:

This happens because the boot interface doesn't have an IP, since we create a bridge and attach the physical interface to it (so that VMs can also get IPs from MAAS.

We should look at the bridge containing the boot interface for an IP.

2022-04-29 15:35:06 metadataserver.api_twisted: [critical] Failed to process status message instantly.
        Traceback (most recent call last):
          File "/usr/lib/python3.8/threading.py", line 870, in run
            self._target(*self._args, **self._kwargs)
          File "/snap/maas/x1/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 821, in worker
            return target()
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
            task()
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
            task()
        --- <exception caught here> ---
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/snap/maas/x1/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 856, in callInContext
            return func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 202, in wrapper
            result = func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.8/site-packages/metadataserver/api_twisted.py", line 561, in _processMessageNow
            self._processMessage(node, message)
          File "/snap/maas/x1/lib/python3.8/site-packages/maasserver/utils/orm.py", line 756, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.8/site-packages/maasserver/utils/orm.py", line 559, in retrier
            return func(*args, **kwargs)
          File "/usr/lib/python3.8/contextlib.py", line 75, in inner
            return func(*args, **kwds)
          File "/snap/maas/x1/lib/python3.8/site-packages/metadataserver/api_twisted.py", line 469, in _processMessage
            _create_vmhost_for_deployment(node)
          File "/snap/maas/x1/lib/python3.8/site-packages/metadataserver/api_twisted.py", line 207, in _create_vmhost_for_deployment
            ip = node.ip_addresses([boot_if])[0]
        builtins.IndexError: list index out of range