x86/hvm: add store and console event channel params to white-list

x86/hvm: add store and console event channel params to white-list

My recent patch "use white-lists for HVM param guest accessibility checks"
(commit 30efda12d63ff536102ed8e580b4c09765683b44) introduced a regression
when saving and restoring Windows guests running Xen Project PV drivers.

The drivers reset the event channel ABI on resume. This has the effect of
closing the store and console local event channel ports created by the
toolstack. The drivers do not currently use the console port but they
do use the store port and thus, prior to reset, they sample the remote
domain and port and then re-bind the channel - yielding a new and possibly
different local port - after reset. They then store the new local port
value in HVM_PARAM_CONSOLE_EVTCHN. That operation now fails and may
lead to a crash, if using a debug build of the drivers, or a barely
functional connection to xentored if not.

This patch adds HVM_PARAM_STORE_EVTCHN and HVM_PARAM_CONSOLE_EVTCHN to
the guest-settable white-list since either of them can be invalidated
by an event channel ABI reset and thus the guest may indeed need to
write new values to them to make them valid again.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>