s390x: fix make standalone

A recent patch broke make standalone. The function find_word is not
available when running make standalone, replace it with a simple grep.

Fixes: 743cacf7 ("s390x: don't run migration tests under PV")
Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20221220175508.57180-1-imbrenda@linux.ibm.com>
Reviewed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>