KVM: guest_memfd: Only "prepare" folios for private pages

When getting a guest_memfd pfn, prepare the folio, i.e. convert its pages
to private, if and only if the page is actually private.  The misnamed
prepare() hook exists specifically to allow x86's SNP to assign pages to
the owning VM in the RMP when mapping private memory into a guest.

Guarding the call will allow renaming the prepare() hook to better reflect
its role, without creating a semantic mess, and will become a hard
requirement once in-place conversion is supported, i.e. when CoCo VMs
support SHARED guest_memfd pages.

For all intents, no functional change intended (the sole arch hook is a nop
for SHARED memory).

Suggested-by: Michael Roth <michael.roth@amd.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
[sean: rewrite changelog to fit the context]
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20260723210811.72720-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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