libxl: wire the libxl_device_nic 'mtu' value into xenstore

libxl: wire the libxl_device_nic 'mtu' value into xenstore

Currently the 'mtu' field of libxl_device_nic objects is effectively ignored:
It is set by libxl__device_nic_setdefault() to a slightly odd default value of
1492 but otherwise ignored.

This patch changes the default value to a more conventional 1500 and modifies
libxl__set_xenstore_nic() to write the value into an 'mtu' node in the
xenstore backend area (if it is a non-default value), as well as a read-only
node of the same name in the frontend area.

The backend node is used to set the value of 'mtu' in
libxl__nic_from_xenstore(), when retrieving the configuration.

NOTE: There is currently no way to set a non-default value of 'mtu', hence
      the backend node is never written. This, however, will be addressed
      by a subsequent patch.

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