There are a couple of reasons for doing so:
- /proc is not OS independent, it's only supported by Linux (there are
  emulations on other systems, but those might differ from the way
  Linux represents data).
- Checking a daemon's state doesn't necessarily mean it's usable.
  Connecting to the socket using “xm info” is much safer.
- Reduce code size.

Reviewed-by: iustinp