chardev: let g_idle_add() be with chardev gcontext

The idle task will be attached to main gcontext even if the chardev
backend is running in another gcontext.  Fix the only caller by
extending the g_idle_add() logic into the more powerful
g_source_attach().  It's basically g_idle_add_full() implementation, but
with the chardev's gcontext passed in.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180104141835.17987-3-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>