ANDROID: trusty-virtio: use struct_size()

Follow the recommended kernel coding style.
See https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments



Test: trusty/vendor/google/aosp/scripts/build.py qemu-generic-arm64-test-debug
Change-Id: Ife4b4f887ec3b3334af15d5f21535727fbf41e98
Signed-off-by: Eric Biggers <ebiggers@google.com>