Move sdl, vnc, curses and cocoa UI into ui/ to cleanup
the root directory. Also remove some unnecessary explicit
targets from Makefile.

aliguori: fix build when srcdir != objdir

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>