qapi: Introduce change-vnc-password

New QMP command to change the VNC password.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
