Let's auto-enable it also when maxmem is specified but no slots are
defined. This will result in us properly creating ACPI srat tables,
indicating the maximum possible PFN to the guest OS. Based on this, e.g.,
Linux will enable the swiotlb properly.

This avoids having to manually force the switolb on (swiotlb=force) in
Linux in case we're booting only using DMA memory (e.g., 2GB on x86-64),
and virtio-mem adds memory later on that really needs the swiotlb to be
used for DMA.

Let's take care of backwards compatibility if somebody has a setup that
specifies "maxram" without "slots".
Reported-by: NAlex Shi <alex.shi@linux.alibaba.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Sergio Lopez <slp@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: qemu-arm@nongnu.org <qemu-arm@nongnu.org>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20200626072248.78761-22-david@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>