bhyve: improve console error reporting on arm64

Currently, on arm64, if bhyve fails to initialize the console,
it falls into assert(), which does not look particularly pretty
for users.

Replace the assert with proper error handling so bhyve prints
a meaningful error message and exits with status code 4 (error).
That matches the behavior on amd64.

Approved by: markj
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D54504