[xen] Wait for and clear XenStore event before receiving data

[xen] Wait for and clear XenStore event before receiving data

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>