xend, 8MB video memory and ballooning

[Samuel Thibault <samuel.thibault@eu.citrix.com>]

Hello,

There's an issue when starting an HVM domain and quickly after, another
domain:

- xend asks the balloon driver to free enough memory for the HVM domain
  plus 8MB for the video memory.
- before the HVM domain actually starts running (e.g. qemu long to
  start), I start another domain, xend asks the balloon driver to free
  enough memory for that other domain, but no more.
- the HVM domain eventually starts, the Cirrus VGA BIOS activates the
  video LFB, and thus qemu tries to call populate physmap in order to
  actually make use of the extra 8MB for the video memory.
  
Problem: the 8MB extra free memory has been swallowed by the other
domain...

(of course, my concern is about the device model stubdomain starting
right after the HVM domain, but I guess that may happen in other cases
too).

Samuel