An insufficient boundary validation in the USB code could lead to an out-of-bounds write on the heap, with data controlled by the caller.
A malicious, privileged software running in a guest VM can exploit the vulnerability to achieve code execution on the host in the bhyve userspace process, which typically runs as root. Note that bhyve runs in a Capsicum sandbox, so malicious code is constrained by the capabilities available to the bhyve process.