xc: deal with xen/evtchn and xen/gntdev device names

xc: deal with xen/evtchn and xen/gntdev device names

This patch makes xc_linux properly deal with:
 1. discovering and creating device nodes if necessary
 2. the new form of xen/<dev> device names soon to be used by the
 kernel

This changes the logic slightly:
 - If a device node already exists with the proper name, then it uses
 it as-is, assuming it has already been correctly created.
 - If the path doesn't exist, or it exists but isn't a device node,
 and
   it has successfully found the major/minor for the device, then
   (re)create the device node.

Since this logic is identical for gntdev and evtchn, make a common
function to handle both.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>