memory: populate FlatView for new address spaces

Even a new address space might have a non-empty FlatView.  In order
to initialize it properly, address_space_init should (a) call
memory_region_transaction_commit after the address space is inserted
into the list; (b) force memory_region_transaction_commit to do something.

This bug was latent so far because all address spaces started empty, including
the PCI address space where the bus master region is initially disabled.
However, the target address space of an IOMMU is usually rooted at
get_system_memory(), which might not be empty at the time the IOMMU is created.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>