A quadratic form is defined as $q(\mathbf{x}) = \mathbf{x}^T A \mathbf{x}$ where $\mathbf{x} = \begin{pmatrix} x \\ y \end{pmatrix}$ and $A = \begin{pmatrix} 2 & -1 \\ -1 & 2 \end{pmatrix}$. Which of the following correctly describes the nature of this quadratic form?