KVM: guest_memfd: make kvm_gmem_prepare_folio() operate on a single struct kvm

KVM: guest_memfd: make kvm_gmem_prepare_folio() operate on a single struct kvm

This is now possible because preparation is done by kvm_gmem_get_pfn()
instead of fallocate().  In practice this is not a limitation, because
even though guest_memfd can be bound to multiple struct kvm, for
hardware implementations of confidential computing only one guest
(identified by an ASID on SEV-SNP, or an HKID on TDX) will be able
to access it.

In the case of intra-host migration (not implemented yet for SEV-SNP,
but we can use SEV-ES as an idea of how it will work), the new struct
kvm inherits the same ASID and preparation need not be repeated.

Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>