Our asprintf() implementation guarantees that strp will be NULL on
allocation failure, but this is not standard behaviour.  Detect errors
by checking for a negative return value instead of a NULL pointer.

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

Older, out-of-tree Xen kernel modules (such as those provided with
SuSE Linux Enterprise Server 11) do not clear the leftover "event
pending" bit when opening an event channel.  Consequently, no event is
ever delivered to indicate that there is information in the XenStore
ring buffer, and the system hangs shortly after loading the
xen-platform-pci kernel module.

Work around this problem by always waiting for the XenStore event
channel to be signalled, and clearing the event before processing the
received data.

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>

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>