PPC: spapr: Remove global variable

Global variables are bad. Let's move spapr_has_graphics into the
machine state struct.

Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/hw/spapr.h b/hw/spapr.h
index 9153f29..fe40e7d 100644
--- a/hw/spapr.h
+++ b/hw/spapr.h
@@ -23,6 +23,7 @@
     int next_irq;
     int rtc_offset;
     char *cpu_model;
+    bool has_graphics;
 } sPAPREnvironment;
 
 #define H_SUCCESS         0