vDPA/ifcvf: implement vdpa_config_ops.get_vq_num_min

IFCVF HW supports operation with vq size less than the max size,
as the spec required.

This commit implements vdpa_config_ops.get_vq_num_min to report
the minimal size of the virtqueues, which gives vDPA framework
a chance to reduce the vring size.

We need at least one descriptor to be functional, but it is better
no less than 64 to meet ceratin performance requirements.
Actually the framework would allocate at least a PAGE for the vq.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240202163905.8834-11-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>