tests: mock qemuInterfaceOpenVhostNet

tests: mock qemuInterfaceOpenVhostNet

This functions contains logic that tries to use vhost for virtio
interfaces, even if <driver name='vhost'/> was not supplied.
In this case, a failure is non-fatal.

On my system, /dev/vhost-net was not accessible to the user running
'make check', but we should not depend on that.

Mock it to prevent accessing /dev/vhost-net and return some predictable
file descriptor numbers instead.

Introduced by commit c1f684e - deprecate QEMU_CAPS_VHOST_NET.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reported-by: Jiří Denemark <jdenemar@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>