test-coroutine: Fix coroutine pool corruption

The test case overwrites the Coroutine object with 0xff as a way to
assert that the coroutine isn't used any more. However, this means that
the coroutine pool now contains a corrupted object and later test cases
may get this corrupted object and crash.

This patch saves the real content of the object and restores it after
completing the test. The only use of the coroutine pool between those
two points is the deletion of co2. As this only means an insertion at
the head of an SLIST (release_pool or alloc_pool), it doesn't access the
invalid list pointers that co1 has during this period.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>