bhyve: improve console error reporting on arm64

[novel]

Summary

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.

[novel]

Relatively simple way to trigger this error is to run kldunload nmdm and then run bhyve -c 1 -o bootrom=/usr/local/share/u-boot/u-boot-bhyve-arm64/u-boot.bin -o console=/dev/nmdmFoobarA test.