qemu-ga: implement win32 guest-set-user-password

Use NetUserSetInfo() to set the user password.

This function is notoriously known to be problematic for users with EFS
encrypted files. But the alternative, NetUserChangePassword() requires
the old password. Nevertheless, The EFS file should be recovered by
changing back to the old password.

Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
