We try to use that command first when setting a VNC/SPICE password. If
that doesn't work we fallback to the legacy VNC only password

Allow an expiry time to be set, if that doesn't work, throw an error
if they try to use SPICE.

Change since v1:
- moved qemuInitGraphicsPasswords to qemu_hotplug, renamed
  to qemuDomainChangeGraphicsPasswords.
- updated what looks like a typo (that appears to work anyway) in
  initial patch from Daniel:
    - ret = qemuInitGraphicsPasswords(driver, vm,
    -                                 VIR_DOMAIN_GRAPHICS_TYPE_SPICE,
    -                                 &vm->def->graphics[0]->data.vnc.auth,
    -                                 driver->vncPassword);
    + ret = qemuInitGraphicsPasswords(driver, vm,
    +                                 VIR_DOMAIN_GRAPHICS_TYPE_SPICE,
    +                                 &vm->def->graphics[0]->data.spice.auth,
    +                                 driver->spicePassword);

Based on patch by Daniel P. Berrange <berrange@redhat.com>.