tests/migration: Skip tests for ppc tcg

PPC tcg seems to be failing migration tests quite regularly;
we believe this is TCG bugs in dirty bit updating; it's
not clear why PPC fails more but lets skip for the moment.

$ ./tests/migration-test
/ppc64/migration/deprecated: OK
/ppc64/migration/bad_dest: Skipping test: kvm_hv not available OK
/ppc64/migration/postcopy/unix: Skipping test: kvm_hv not available OK
/ppc64/migration/precopy/unix: Skipping test: kvm_hv not available OK

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-id: 20180706143105.93472-1-dgilbert@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>