evtchn/fifo: map correct pages when guest is HVM

evtchn/fifo: map correct pages when guest is HVM

If a HVM guest attempts to use the FIFO-based ABI it will not receive
any events and destroying the guest may crash Xen or trigger an assert
when attempting to unmap a control block page.  This occurs because
Xen maps the wrong page for both the control blocks and the event
arrays.

In map_guest_page(), use the MFN of the guest's page and not the GFN
when calling map_domain_page_global().

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>