content: reserve device ID 48 for Virtio USB host controller device

[aiscyriac]

The Virtio USB host controller device allows USB peripheral devices connected
to a host port to be connected to the guest. The host controller device can
have one or more ports, and it allows the guest to have virtual ports corresponding
to physical ports in the host.

This patch requests device ID 48, as device ID 47 has been requested for virtio CPU baloon device (#187).