tools/libs/call: Describe return values and error semantics for xencall*

tools/libs/call: Describe return values and error semantics for xencall*

This behaviour has been confirmed by inspection on:

 - Linux
 - NetBSD & FreeBSD (NB: hcall->retval is the hypercall return value
   only for values >= 0. For negative values the underlying privcmd
   driver translates the value from Xen to {Net,Free}BSD errno space
   and returns it as the result of the ioctl, which becomes
   ret=-1/errno=EFOO in userspace)
 - MiniOS (which takes care of errno in this library)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>