char: use qemu_chr_fe* functions with CharBackend argument

This also switches from qemu_chr_add_handlers() to
qemu_chr_fe_set_handlers(). Note that qemu_chr_fe_set_handlers() now
takes the focus when fe_open (qemu_chr_add_handlers() did take the
focus)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-16-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>