Capture the error message of bhyve about vm-bhyve

[churchers]

It seems adding the following to the end of the bhyve command in lib/vm-run captures errors

 >"${vm_dir}/${_name}/bhyve.log" 2>&1

I'm not sure yet whether it's a good idea to add this as a standard function, or possibly as part of a 'debug' mode that can be switched on if needed.

[churchers]

I've added an option to the configuration file:

debug="yes"

With this enabled, all the output (STDOUT & STDERR) is written to ${vm_dir}/guest/bhyve.log. I think this is a reasonable compromise to allow the output to be captured if required, with the ability to just leave it there if you always want to capture bhyve output. By default we run as normal.

[WanpengQian]

It works. thank you!