core: Extend entry_valid paging callback to check for flags

core: Extend entry_valid paging callback to check for flags

Generalize the entry_valid callback of paging_structures to test for a
given set of flags, not only the present flag. This will allow us to
validate the access mode on guest and 2nd-level page table walks. For
now we just continue to test PAGE_PRESENT_FLAGS.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>