pagecache: don't assume ARC buffer virtual addresses are in 1:1 map

Using the debug allocator, ARC cache virtual addresses can be outside the
1:1 map, so we have to compare using physical addresses.

Fixes total failure with conf-debug_memory=1.

Suggested by Pawel.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>