vq log eventfd context pointer needs to be initialized, otherwise
operation may fail or oops if log is enabled but log eventfd not set by
userspace.  When log_ctx for device is created, it is copied to the vq.
This reset was missing.
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>