qemu: monitor: Produce better errors on monitor hangup

qemu: monitor: Produce better errors on monitor hangup

Change the monitor error code to add the ability to access the qemu log
file using a file descriptor so that we can dig in it for a more useful
error message. The error is now logged on monitor hangups and overwrites
a possible lesser error. A hangup on the monitor usualy means that qemu
has crashed and there's a significant chance it produced a useful error
message.

The functionality will be latent until the next patch.