x86, hvm: stdvga improvements

x86, hvm: stdvga improvements

* enables linear framebuffer and VBE 3.0 extensions.
* add a number of new resolutions, up to 2560x1600, and including
* widescreen modes common to modern displays.
* Introduce configurable video ram size through the "videoram" config
* file option, shrinking default video ram used to 4 Megs (thus
* eliminating 4 wasted megs/VM that have been the case until now), but
* allowing users to specify up to 16 Megs of vram, to support the
* extreme high resolutions such as 2560x1600x32.

Signed-off-by: Trolle Selander <trolle.selander@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>