xenserver: Retrieve vSwitch version from binary in xsconsole

xenserver: Retrieve vSwitch version from binary in xsconsole

The xsconsole plugin shows status information about Open vSwitch.  The
version information was retrieved from XAPI, but this could cause
problems.  The most easily reproduced is to make a XenServer part of a
pool, then remove it.  The version string is no longer in the
XenServer's local XAPI view, so it reports "<unknown>".  A more direct
way to get the information is to directly query the binary, which is
what this commit does.

Bug #1626