make vga screen_dump use DisplayState properly

Hi all,
currently the vga screen_dump code doesn't use the DisplayState
interface properly and tries to replace it temporarily while taking the
screenshot.
A better approach is to register a DisplayChangeListener, call
vga_hw_update, and finally write the ppm in the next call from dpy_update.

Testing is appreciated.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>