vdpa: add new attribute VDPA_ATTR_DEV_MIN_VQ_SIZE

This attribute advertises the min value of virtqueue size. The value is
1 by default.

Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Link: https://lore.kernel.org/r/2bbc417355c4d22298050b1ba887cecfbde3e85d.1635493219.git.wuzongyong@linux.alibaba.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>