x86/p2m: Fix paging_gva_to_gfn() for nested virt

x86/p2m: Fix paging_gva_to_gfn() for nested virt

nestedhap_walk_L1_p2m() takes guest physical addresses, not frame numbers.
This means the l2 input is off-by-PAGE_SHIFT, as is the l1 value eventually
returned to the caller.

Delete the misleading comment as well.

Fixes: bab2bd8e222de ("xen/nested_p2m: Don't walk EPT tables with a regular PT walker")
Reported-by: Tamas K Lengyel <tamas@tklengyel.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>