Change alignment of virtiofs structs to maximum alignment of all fields.

Alignment of the virtiofs struct was previously based on the alignment
of only the first field. Changed to explicitly use the maximum alignment of all of
the fields and added assertions to ensure this manually-allocated layout
matches the compiler-generated layout.

Fixes hermit-os#691

Signed-off-by: joannejchen <chenjjoanne@gmail.com>