Do not hang on full PTY

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-char.c

@@ -1402,6 +1402,7 @@ static CharDriverState *qemu_chr_open_pty(const char *id,
     }
 
     close(slave_fd);
+    qemu_set_nonblock(master_fd);
 
     chr = qemu_chr_alloc();