[lotest] Discard packets arriving on the incorrect network device

Commit 24bbaf6 ("[lotest] Allow loopback testing on shared networks")
introduced a regression in which loopback testing packets would be
accepted from any network device.  This produces unexpected results,
such as VLAN loopback testing succeeding even when incorrectly using
the underlying trunk device as either transmitter or receiver.

Fix by discarding any loopback testing packets which arrive on a
network device other than the current loopback testing receiver.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[xen] Import selected public headers

Import selected headers from the xen/include/public directory of the
Xen repository at git://xenbits.xen.org/xen.git

The script ./include/xen/import.pl can be used to automatically import
any required headers and their dependencies (in a similar fashion to
./include/ipxe/efi/import.pl).  Trailing whitespace is stripped and an
appropriate FILE_LICENCE declaration is added to each header file.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[xen] Add basic support for PV-HVM domains

Add basic support for Xen PV-HVM domains (detected via the Xen
platform PCI device with IDs 5853:0001), including support for
accessing configuration via XenStore and enumerating devices via
XenBus.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[xen] Add support for Xen netfront virtual NICs

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge branch 'master' into osimages