Implement infrastracture for mocking up QEMU capabilities cache

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>