x86/mm: Fix deadlock between p2m and event channel locks.

x86/mm: Fix deadlock between p2m and event channel locks.

The hvm io emulation code holds the p2m lock for the duration of the
emulation, which may include sending an event to qemu. On a separate path,
map_domain_pirq grabs the event channel and p2m locks in opposite order.

Fix this by ensuring liveness of the ram_gfn used by io emulation, with a
page ref.

Reported-by: "Hao, Xudong" <xudong.hao@intel.com>
Signed-off-by: "Hao, Xudong" <xudong.hao@intel.com>
Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>