pseries: disable migration-test if /dev/kvm cannot be used

On ppc64, migration-test only works with kvm_hv, and we already
have a check to verify the module is loaded.

kvm_hv module can be loaded in memory and /sys/module/kvm_hv exists,
but on some systems (like build systems) /dev/kvm can be missing
(by administrators choice).

And as kvm_hv exists test-migration is started but QEMU falls back to
TCG because it cannot be used:

Could not access KVM kernel module: No such file or directory
failed to initialize KVM: No such file or directory
Back to tcg accelerator

And as the test is done with TCG, it fails.

As for s390x, we must check for the existence and the access rights
of /dev/kvm.

Reported-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20191120170955.242900-1-lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>