g_source_attach can return any value between 1 and UINT_MAX if you let
QEMU run long enough. However, qemu_chr_fe_add_watch can also return
a negative errno value when the device is disconnected or does not
support chr_add_watch. Change it to return zero to avoid overloading
these values.
Fix the cadence_uart which asserts in this case (easily obtained with
"-serial pty").
Tested-by: Bret Ketchum <bcketchum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
6f1de6b70d