Do not activate boot=on on devices when not using KVM

  Basically the 'boot=on' boot selection device is something present in
KVM but not in upstream QEmu, as a result if we boot a QEmu domain
without KVM acceleration we must disable boot=on ... even if the front
end kvm binary expose that capability in the help page.

* src/qemu/qemu_conf.c: in qemudBuildCommandLine if -no-kvm
  is passed, then deactivate QEMUD_CMD_FLAG_DRIVE_BOOT

src/qemu/qemu_conf.c

@@ -3707,9 +3707,17 @@ int qemudBuildCommandLine(virConnectPtr conn,
      * 2. The qemu binary has the -no-kvm flag
      */
     if ((qemuCmdFlags & QEMUD_CMD_FLAG_KVM) &&
-        def->virtType == VIR_DOMAIN_VIRT_QEMU)
+        def->virtType == VIR_DOMAIN_VIRT_QEMU) {
         disableKVM = 1;
 
+        /*
+         * do not use boot=on for drives when not using KVM since this
+         * is not supported at all in upstream QEmu.
+         */
+        if (qemuCmdFlags & QEMUD_CMD_FLAG_DRIVE_BOOT)
+            qemuCmdFlags -= QEMUD_CMD_FLAG_DRIVE_BOOT;
+    }
+
     /* Should explicitly enable KVM if
      * 1. Guest domain is 'kvm'
      * 2. The qemu binary has the -enable-kvm flag