Xen 4.4 includes the device "device/suspend/event-channel" which does
not have a "backend" key.  This currently causes the entire XenBus
device tree probe to fail.

Fix by skipping probe attempts for device types for which there is no
iPXE driver.

Debugged-by: Eytan Heidingsfeld <eytanh@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

Relicense files for which I am the sole author (as identified by
util/relicense.pl).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Under some circumstances (e.g. if iPXE itself is booted via iSCSI, or
after an unclean reboot), the backend may not be in the expected
InitWait state when iPXE starts up.

There is no generic reset mechanism for Xenbus devices.  Recent
versions of xen-netback will gracefully perform all of the required
steps if the frontend sets its state to Initialising.  Older versions
(such as that found in XenServer 6.2.0) require the frontend to
transition through Closed before reaching Initialising.

Add a reset mechanism for netfront devices which does the following:

 - read current backend state

 - if backend state is anything other than InitWait, then set the
   frontend state to Closed and wait for the backend to also reach
   Closed

 - set the frontend state to Initialising and wait for the backend to
   reach InitWait.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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>