No need to keep explicit_fe_open around if it affects only a
qemu_chr_fe_set_handlers(). Use an additional argument instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-24-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>