vhost: initialize log eventfd context pointer

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>