libxl: fix build on platforms where openpty's parameters are not const.

libxl: fix build on platforms where openpty's parameters are not const.

Such as NetBSD. Fixes this build error:

libxl_aoutils.c: In function 'libxl__openptys':
libxl_aoutils.c:281:13: error: passing argument 4 of 'openpty' discards
qualifiers from pointer target type
/usr/include/util.h:92:6: note: expected 'struct termios *' but argument
is of type 'const struct termios *'
libxl_aoutils.c:281:13: error: passing argument 5 of 'openpty' discards
qualifiers from pointer target type
/usr/include/util.h:92:6: note: expected 'struct winsize *' but argument
is of type 'const struct winsize *'

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Acked-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>