x86/shadow: slightly consolidate sh_unshadow_for_p2m_change() (part III)

x86/shadow: slightly consolidate sh_unshadow_for_p2m_change() (part III)

In preparation for reactivating the presently dead 2M page path of the
function, also deal with the case of replacing an L1 page table all in
one go. Note that the prior comparing of MFNs to bypass the removal of
shadows was insufficient (but kind of benign, for being dead code so
far) - at the very least the R/W bit also needs considering there (to be
on the safe side, compare the full [virtual] PTEs).

While adjusting the first conditional in the loop for the use of the new
local variable "nflags", also drop mfn_valid(): If anything we'd need to
compare against INVALID_MFN, but that won't come out of l1e_get_mfn().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>