KVM: MMU: Disassociate direct maps from guest levels

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: Avi Kivity <avi@redhat.com>