All DisplayType values are just UI options that don't affect any
hardware emulation code, except for DT_NOGRAPHIC. Replace
DT_NOGRAPHIC with DT_NONE plus a new "-machine graphics=on|off"
option, so hardware emulation code don't need to use the
display_type variable.

Cc: Michael Walle <michael@walle.cc>
Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>