mempool: replace physical address by a memzone pointer

mempool: replace physical address by a memzone pointer

Storing the pointer to the memzone instead of the physical address
provides more information than just the physical address: for instance,
the memzone flags.

Moreover, keeping the memzone pointer will allow us to free the mempool
(this is done later in the series).

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>