pmb.qemu: add --no-kvm to disable KVM even when available (!1886)

To test QEMU's CPU emulation it is useful to have a switch to disable
KVM, even when it is available (and potentially working fine).

Add --no-kvm for that purpose.

pmb/parse/arguments.py

@@ -123,6 +123,10 @@ def arguments_qemu(subparser):
     ret.add_argument("--flavor", help="name of the kernel flavor (run 'pmbootstrap flasher list_flavors'"
                      " to get a list of all installed flavors")
 
+    ret.add_argument("--no-kvm", dest="qemu_kvm", default=True, action='store_false',
+                     help="Avoid using hardware-assisted virtualization with KVM "
+                     "even when available (SLOW!)")
+
     ret.add_argument("--tablet", dest="qemu_tablet", action='store_true',
                      default=False, help="Use 'tablet' instead of 'mouse' input "
                      "for QEMU. The tablet input device automatically grabs/releases "

pmb/qemu/run.py

@@ -144,7 +144,7 @@ def command_qemu(args, arch, img_path):
 
     # Kernel Virtual Machine (KVM) support
     native = args.arch_native == args.deviceinfo["arch"]
-    if native and os.path.exists("/dev/kvm"):
+    if args.qemu_kvm and native and os.path.exists("/dev/kvm"):
         command += ["-enable-kvm"]
     else:
         logging.info("WARNING: QEMU is not using KVM and will run slower!")