mem: fix ivshmem in malloc heap

After the changes introduced by Dynamic Memzones, all the memsegs were
added to the malloc heap during init.

Those changes did not account for IVSHMEM memsegs which should not be
added to the malloc heap as part of available memory.

Fixes: fafcc11985a2 ("mem: rework memzone to be allocated by malloc")

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>