bhyve: Avoid passing a possible garbage pointer to free().

All of the error paths in pci_vtcon_sock_add free the sock pointer.
However, sock is not initialized until part way through the function.
An early error would pass stack garbage to free().

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37491

(cherry picked from commit bc928800723b65daa9b005bec4ffd8ad8c781a09)