test: SKip TestMachinesDisks.testAddDiskNFS on RHEL 10

RHEL 10 got a nasty kernel oops [1] which unceremoniously reboots the VM
without leaving any journal trace, only the QEMU console shows it.

It completely breaks Testing Farm runs (it cannot recover from reboots)
and also breaks our own CI in nasty ways, as after the reboot the
nondestructive recovery fails in all kinds of ways.

Fixes cockpit-project#1900

[1] https://issues.redhat.com/browse/RHEL-67841

test/check-machines-disks

@@ -848,6 +848,8 @@ class TestMachinesDisks(machineslib.VirtualMachinesCase):
         # iscsi disks are attached.
         m.execute("virsh detach-disk subVmTest1 --target vdb")
 
+    @testlib.skipImage("triggers kernel oops, https://issues.redhat.com/browse/RHEL-67841",
+                       "rhel-10*", "centos-10")
     def testAddDiskNFS(self):
         b = self.browser
         m = self.machine