xen Error: Device 2050 (vbd) could not be connected. Backend device not found.

Problem
NetBSD dom0 is unable to launch more domUs. Attempting to launch more domUs results in the following error:

xen Error: Device 2050 (vbd) could not be connected. Backend device not found.

Solution
The system ran out of loop devices. Create more vnode disks using the following command:

/dev/MAKEDEV vnd1
/dev/MAKEDEV vnd2
...and so on