char: Fix return type of qemu_chr_fe_add_watch()

qemu_chr_fe_add_watch() can return negative errors, therefore it must
not have an unsigned return type. For consistency with other
qemu_chr_fe_* functions, this uses a standard C int instead of glib
types.

In situations where qemu_chr_fe_add_watch() is falsely assumed to have
succeeded, the serial ports would go into a state where it never becomes
ready for transmitting more data; this is fixed by this patch.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
