virtio_mmio: fix one typo

fix the typo 'there is are' to 'there are'.

Signed-off-by: Xianting Tian <xianting_tian@126.com>
Link: https://lore.kernel.org/r/1612615619-8128-1-git-send-email-xianting_tian@126.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>