Add an argument to allow the demand paging test to work on larger and smaller guest sizes. Signed-off-by: Ben Gardon <bgardon@google.com> [Rewrote parse_size() to simplify and provide user more flexibility as to how sizes are input. Also fixed size overflow assert.] Signed-off-by: Andrew Jones <drjones@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>