char: change qemu_chr_fe_add_watch to return unsigned

char: change qemu_chr_fe_add_watch to return unsigned

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 <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>