lib/alloc_page: Wire up FLAG_DONTZERO

Memory allocated without FLAG_DONTZERO will now be zeroed before being
returned to the caller.

This means that by default all allocated memory is now zeroed, restoring the
default behaviour that had been accidentally removed by a previous commit.

Fixes: 8131e91a ("lib/alloc_page: complete rewrite of the page allocator")
Reported-by: Nadav Amit <nadav.amit@gmail.com>

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20210115123730.381612-11-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>