Separate the allocation of shadow pages from their initialization.  This
is in preparation for splitting huge pages outside of the vCPU fault
context, which requires a different allocation mechanism.

No functional changed intended.
Reviewed-by: NPeter Xu <peterx@redhat.com>
Signed-off-by: NDavid Matlack <dmatlack@google.com>
Message-Id: <20220119230739.2234394-15-dmatlack@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>