tests: check if qemu-nbd is still alive before waiting

tests: check if qemu-nbd is still alive before waiting

If the qemu-nbd UNIX socket has not shown up, the tests will sleep a bit
and then check again repeatedly for up to 30 seconds. This is pointless
if the qemu-nbd process has quit due to an error, so check whether the
pid is still alive before waiting and retrying.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20181116155325.22428-5-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>