bhyve: avoid buffer overflow in pci_vtcon_control_send

This is a follow-up to the fix for HYP-19, addressing another condition
where an overflow might still occur. (Spotted by jhb@, thanks!)

Reported by: Synacktiv
Reviewed by: markj
Security: HYP-19
Sponsored by: Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46882