bhyve: avoid buffer overflow in pci_vtcon_control_send

The program copies an input buffer to an output buffer without verifying
that the size of the input buffer is less than the size of the output
buffer, leading to a buffer overflow.

Inside the function pci_vtcon_control_send, the length of the iov buffer
is not validated before copy of the payload.

Reported by: Synacktiv
Reviewed by: markj
Security: HYP-19
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46105