vDPA/ifcvf: get_config_size should return a value no greater than dev implementation

Drivers must not access a BAR outside the capability length,
and for a virtio device, ifcvf driver should not report any non-standard
capability contents to the upper layers.

Function ifcvf_get_config_size() is introduced here to return a safe value
of the device config capability size.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20220722115309.82746-2-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>