A library has to be built with -flat_namespace to get all references to
global symbols indirected. That can also be achieved with two-level
namespace interposition but we're not using explicit symbol
interposition since it's more verbose and requires massive changes to
the mocks.

This provides a way to interpose a mock for virQEMUCapsProbeHostCPU from
qemucpumock and fixes domaincapstest on macOS.
Signed-off-by: NRoman Bolshakov <r.bolshakov@yadro.com>