memory: Move FlatView allocation to a helper

This moves a FlatView allocation and initialization to a helper.
While we are nere, replace g_new with g_new0 to not to bother if we add
new fields in the future.

This should cause no behavioural change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-4-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>