Do not add drive 'boot=on' param when a kernel is specified

libvirt-tck was failing several domain tests [1] with qemu 0.14, which
is now less tolerable of specifying 2 bootroms with the same boot index [2].

Drop the 'boot=on' param if kernel has been specfied.

[1] https://www.redhat.com/archives/libvir-list/2011-February/msg00559.html
[2] http://lists.nongnu.org/archive/html/qemu-devel/2011-02/msg01892.html

src/qemu/qemu_command.c

@@ -3173,7 +3173,7 @@ qemuBuildCommandLine(virConnectPtr conn,
         int bootCD = 0, bootFloppy = 0, bootDisk = 0;
 
         /* If QEMU supports boot=on for -drive param... */
-        if (qemuCmdFlags & QEMUD_CMD_FLAG_DRIVE_BOOT) {
+        if (qemuCmdFlags & QEMUD_CMD_FLAG_DRIVE_BOOT && !def->os.kernel) {
             for (i = 0 ; i < def->os.nBootDevs ; i++) {
                 switch (def->os.bootDevs[i]) {
                 case VIR_DOMAIN_BOOT_CDROM: