Similar to other handle_aiocb_* functions, handle_aiocb_ioctl needs to cater
for the possibility that ioctl is interrupted by a signal.  Otherwise, the
I/O is incorrectly reported as a failure to the guest.

Reported-by: Gordon Watson <gwatson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>