Use resizeable ram API so we can painlessly extend ROMs in the
future.  Note: migration is not affected, as we are
not actually changing the used length for RAM, which
is the part that's migrated.

Use this in acpi: reserve x16 more RAM space.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>