xenapi: revisit error handling around calls to agent

Now we have settings to hint if the agent is present or not,
and by default we do not check for the agent,
if the agent is not responding to our calls for its version,
we can fail the build.

In environments that need the agent to inject the networking,
you really want the agent to fail if it is not present.
If the agent did not inject the networking, the server will
have no networking.

However, we can still leave the agent upgrade to silently
fail, as the agent should be backwards compatible.

fixes bug 1188540

Change-Id: I8acdabd8d2bd24b088dad3cd4abec300d0ada3fb
Merge "xenapi: revisit error handling around calls to agent"