spice: call qemu_spice_set_passwd() during init

Don't call SPICE API directly to set password given in command line, but
use the internal API, saving password for later calls.

This solves losing password when changing expiration in qemu monitor.

https://bugzilla.redhat.com/show_bug.cgi?id=1138639

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
