vga: do not resize the screen on hw_invalidate

Hi all,
currently vga always resizes the screen when vga_hw_invalidate is called
while this is not required and all the other graphic emulators don't.
This patch fixes it, making vga invalidate behaviour consistent with the
other emulated devices.

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