iommu-debug: allocate test_virt_addr when reading it

Change to allocate test_virt_addr of size 1 MB when reading
test_virt_addr for the first time. This will ensure allocation
to be made when tests are exercised and this doesn't add in
static boot time allocation.

Change-Id: If8babd47f52d954fb62b7c44f51268ee9bb6b0c0
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>