Ever since commit c5a00350 the libxl parser invokes the emulator
to probe which device model to use.

Commit b90c4b5f introduced a workaround that used a stable path
which was very likely to result in the answer matching the default.
However the test is still affected by the host state and the binary
gets invoked if present.

Mock the libxlDomainGetEmulatorType function to stop wasting CPU
cycles every time a 'make check' is run on a system with xen installed.

For example xlconfigtest gets faster by 90 %
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Fixes: b90c4b5fReviewed-by: NJim Fehlig <jfehlig@suse.com>