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

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

Pull common checks out of the switch(). This includes extending a
_PAGE_PRESENT check to L1 as well, which presumably was deemed redundant
with p2m_is_valid() || p2m_is_grant(), but I think we are better off
being explicit in all cases. Note that for L2 (or higher) the grant
check isn't strictly necessary, as grants are only ever single pages.
Leave a respective assertion.

With _PAGE_PRESENT checked uniformly, the suspicious mfn_valid(omfn)
checks can be dropped rather than moved/folded - if anything we'd need
to compare against INVALID_MFN, but that won't come out of l1e_get_mfn().

For L1 replace the moved out condition with a PTE comparison: There's
no need for any update or flushing when the two match.

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