memory: Create FlatView directly

memory: Create FlatView directly

This avoids usual memory_region_transaction_commit() which rebuilds
all FVs.

On POWER8 with 255 CPUs, 255 virtio-net, 40 PCI bridges guest this brings
down the boot time from 25s to 20s and reduces the amount of temporary FVs
allocated during machine constructon (~800000 -> ~640000) and amount of
temporary dispatch trees (~370000 -> ~300000), the total memory footprint
goes down (18G -> 17G).

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-18-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>