libxl: Earlier detection of not running on Xen

libxl: Earlier detection of not running on Xen

Detect early on in libxl driver initialization if the driver
should be loaded at all, avoiding needless initialization steps
that only have to be undone later.  While at it, move the
detection to a helper function to improve readability.

After detecting that the driver should be loaded, subsequent
failures such as initializing the log stream, allocating libxl
ctx, etc. should be treated as failure to initialize the driver.