config: use 'qemux86-64' instead of 'qemu' as default for KAS_MACHINE

'qemu' is not a valid machine. So, let's use the default one defined in
poky -> 'qemux86-64' [1]

Fixes:

ERROR:  OE-core's config sanity checker detected a potential misconfiguration.
    Either fix the cause of this error or at your own risk disable the checker (see sanity.conf).
    Following is the list of potential problems / advisories:

    MACHINE=qemu is invalid. Please set a valid MACHINE in your local.conf, environment or other configuration file.

[1] - https://git.yoctoproject.org/cgit.cgi/poky/tree/meta-poky/conf/local.conf.sample#n37

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/config.py

@@ -144,7 +144,7 @@ class Config:
             Returns the machine
         """
         return os.environ.get('KAS_MACHINE',
-                              self._config.get('machine', 'qemu'))
+                              self._config.get('machine', 'qemux86-64'))
 
     def get_distro(self):
         """