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: NBret Ketchum <bcketchum@gmail.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>