rust: prefer NonNull::new to assertions

rust: prefer NonNull::new to assertions

Do not use new_unchecked; the effect is the same, but the
code is easier to read and unsafe regions become smaller.
Likewise, NonNull::new can be used instead of assertion and
followed by as_ref() or as_mut() instead of dereferencing the
pointer.

Suggested-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>