rust: do not use --no-size_t-is-usize

rust: do not use --no-size_t-is-usize

This is not necessary and makes it harder to write code that is
portable between 32- and 64-bit systems: it adds extra casts even
though size_of, align_of or offset_of already return the right type.

Reviewed-by: Junjie Mao <junjie.mao@hotmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>