shadow dirty-VRAM: avoid multiple remove_all_mappings calls.

shadow dirty-VRAM: avoid multiple remove_all_mappings calls.

sh_remove_all_mappings() will walk roughly half of the shadow L1
tables for each MFN it's called with; calling it for every MFN in a
guest's framebuffer can be _very_ expensive, especially with the
shadow lock held across the whole operation.  Avoid that by just
blowing away all the shadows.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>