char-stdio: Restore blocking mode of stdout on exit

char-stdio: Restore blocking mode of stdout on exit

qemu_chr_open_fd() sets stdout into non-blocking mode. Restore the old
fd flags on exit to avoid breaking unsuspecting applications that run on
the same terminal after qemu and don't expect to get EAGAIN.

While at at, also ensure term_exit is called once (at the moment it's
called both from char_stdio_finalize() and as the atexit() hook.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2423
Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Link: https://lore.kernel.org/r/20240703190812.3459514-1-maxtram95@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>