kvm: selftests: introduce and use more page size-related constants

Clean up code that was hardcoding masks for various fields,
now that the masks are included in processor.h.

For more cleanup, define PAGE_SIZE and PAGE_MASK just like in Linux.
PAGE_SIZE in particular was defined by several tests.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>