xenconsoled: portability fixes:

xenconsoled: portability fixes:
 - Use openpty(), which does the same as the sequence
   of open(), grantpt(), unlockpt(), ptsname(), tcgetattr()
   simplifies code
 - Check return code from tcsetattr()
 - sprintf() -> snprintf()
 - OpenBSD lacks POSIX grantpt() and unlockpt()
   requires use of openpty()
 - Solaris lacks POSIX openpty() via feedback from SUN (John Levon)
   implement openpty() for Solaris, tested and ok'd by SUN (John
   Levon)

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>