Make python mandatory

The QEMU build depends on Python so make it an explicit requirement.

Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1197,13 +1197,9 @@ if test "$solaris" = "yes" ; then
   fi
 fi
 
-if test "$guest_agent" != "no" ; then
-  if has $python; then
-    :
-  else
+if ! has $python; then
   echo "Python not found. Use --python=/path/to/python"
   exit 1
-  fi
 fi
 
 if test -z "$target_list" ; then