tools/hotplug/Linux: modify set_mtu() to optionally use a configured value...

tools/hotplug/Linux: modify set_mtu() to optionally use a configured value...

...and also inform the frontend.

The set_mtu() function in xen-network-common.sh currently sets the backend
vif MTU to match that of the bridge.

A prior patch added code into libxl such that a tools-configured 'mtu' value
may be present in the xenstore backend area. If the node is present in
xenstore then it should be authoritative. Hence set_mtu() is modified to only
read the MTU of the bridge if it is not present.

The function is also modified to write whatever value it applies to the
backend vif into the xenstore frontend area where is may then be used to
configure the frontend network stack.

NOTE: There is also a small modification replacing '$mtu' with '${mtu}'
      for style consistency.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>