Always use nonblocking mode for qemu_chr_open_fd.

Always use nonblocking mode for qemu_chr_open_fd.

The rest of qemu assumes that IO operations on a CharDriverState do
not block.  Currently there are a couple of cases where such a driver
was set up but the calls to set nonblocking mode were missing:
 * qemu_chr_open_pty
 * qemu_chr_open_pipe
 * qemu_chr_open_stdio

This is fixed by adding two calls to socket_set_nonblock to
qemu_chr_open_fd.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>