xen/arm: Restrict access to most HVM_PARAM's

xen/arm: Restrict access to most HVM_PARAM's

ARM currently has no restrictions on toolstack and guest access to the entire
HVM_PARAM block.  As the monitor feature isn't under security support, this
doesn't need an XSA.

The CALLBACK_IRQ and {STORE,CONSOLE}_{PFN,EVTCHN} details are only exposed
read-only to the guest, while MONITOR_RING_PFN is restricted to only toolstack
access.  No other parameters are used.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien@xen.org>