vDPA/ifcvf: detect and report max allowed vq size

Rather than a hardcode, this commit detects
and reports the max value of allowed size
of the virtqueues

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20230612151420.1019504-3-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>