net/xenvirt: fix build after mempool changes

net/xenvirt: fix build after mempool changes

The field elt_va_start has been removed from the mempool structure,
and it was not replaced in xenvirt.

Fix this by getting the mempool objects address by using the address of
the first memory chunk list.

Note that it won't work with mempool composed of several chunks,
but it was already the case before.

Fixes: 84121f197187 ("mempool: store memory chunks in a list")

Reported-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>