xen: Adds a cap to the number of map cache entries.

Adds a cap to the number of map cache entries. This prevents the map
cache from overwhelming system memory.

I also removed the bitmap macros and #included bitmap.h instead.

Signed-off-By: John Baboval <john.baboval@virtualcomputer.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
