caps: use -device for primary video when qemu >=1.6

https://bugzilla.redhat.com/show_bug.cgi?id=981094
The commit 0ad9025ef introduce qemu flag QEMU_CAPS_DEVICE_VIDEO_PRIMARY
for using -device VGA, -device cirrus-vga, -device vmware-svga and
-device qxl-vga. In use, for -device qxl-vga, mouse doesn't display
in guest window like the desciption in above bug.
This patch try to use -device for primary video when qemu >=1.6 which
contains the bug fix patch

(cherry picked from commit e3f2686bdf6c94f658d8645c32a6039692753411)