CA-107702: Ensure that networkd explicitly configures all physical interfaces with IP in dom0

This is done by marking the interfaces as "persistent" in networkd. Previously, only bridge
interfaces (xenbr*, xapi*) with IP configuration were made persistent. However, their associated
physical interfaces (eth0 etc.) should receive the same treatment, so that attributes such as the
MTU are configured on these as well.

When a host starts, interfaces with IP config need to be configured before xapi can start,
especially on pool slaves, or if HA is enabled.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
CA-109300: nm: correctly set persistence on parent bridge of VLAN

A VLAN bridge is never "persistent" (i.e. it is never brought up by
networkd on startup), while the parent bridge of the VLAN may be
persistent (in case it has a management IP address).

The bug was that, when creating a VLAN, it set the persistence of
the parent bridge equal to the persitence of the VLAN.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1483 from siddharthv/HFX-868

HFX-868: MTU Fix