Implement infrastracture for mocking up QEMU capabilities cache
    
    The main purpose of this patch is to introduce test mode to
    virQEMUCapsCacheLookup(). This is done by adding a global variable, which
    effectively overrides binary name. This variable is supposed to be set by
    test suite.
    
    The second addition is qemuTestCapsCacheInsert() function which allows the
    test suite to actually populate the cache.
    
    Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
    Signed-off-by: Ján Tomko <jtomko@redhat.com>