xsconsole: detect when xapi failed to start and display it

Here's an enhancement suggestion, that we'd probably implement at some point in XCP-ng, and contribute back if/when an external contribution process is opened at some point for xsconsole.

Context: regularly, users show up on forums saying "My host has no IP address", showing a screenshot from xsconsole where, indeed, you see no IP address. Usually it turns out that this is not the real problem. The real problem is that XAPI is not running so xsconsole can't get the IP address from it. The server does have an IP address that can be reached via SSH.

Enhancement suggestion: when XAPI is not running, display a big visible warning in xsconsole stating it. Bonus points if the error that caused it is displayed, but that's probably not easy to add.

Thanks in advance for considering this suggestion!