alsaaudio: Remove unused error handling of qemu_set_fd_handler

The function cannot fail, so the check is superfluous.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1433400324-7358-10-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
