lib: add function to request migration

Migration tests can ask migrate_cmd to migrate them to a new QEMU
process. Requesting migration and waiting for completion is hence a
common pattern which is repeated all over the code base. Add a function
which does all of that to avoid repeating the same pattern.

Since migrate_cmd currently can only migrate exactly once, this function
is called migrate_once() and is a no-op when it has been called before.
This can simplify the control flow, especially when tests are skipped.

Suggested-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20221212111731.292942-2-nrb@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>