ivshmem: Check ivshmem_read() size argument

The third argument to the fd_read() callback implemented by
ivshmem_read() is the number of bytes, not a flags field.  Fix this and
check we received enough bytes before accessing the buffer pointer.

Cc: Cam Macdonell <cam@cs.ualberta.ca>
Reported-by: Sebastian Krahmer <krahmer@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
[AF: Handle partial reads via FIFO]
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
