Direct maps are linear translations for a section of memory, used for
real mode or with large pages.  As such, they are independent of the guest
levels.

Teach the mmu about this by making page->role.glevels = 0 for direct maps.
This allows direct maps to be shared among real mode and the various paging
modes.
Signed-off-by: NAvi Kivity <avi@redhat.com>