devices: virtio: iommu: assign a sensible default value to page_size_mask
devices: virtio: iommu: assign a sensible default value to page_size_mask

Assign a sensible default value to page_size_mask and reject the empty
page_size_mask case.

BUG=b:185084350
TEST=Boot a crosvm guest with vIOMMU enabled

Change-Id: Id711c9ff25265f707618a58eb212fd5ca16383e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3580372
Reviewed-by: David Stevens <2f87ba95ff5d327328bd68c2e86a4d9f2ba99bd0@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: David Stevens <2f87ba95ff5d327328bd68c2e86a4d9f2ba99bd0@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm