memory: Use transaction_begin/commit also for single-step operations

Wrap also simple operations consisting only of a single step with
memory_region_transaction_begin/commit. This allows to perform
additional steps like coalesced MMIO flushing from a single place.

This requires dropping some micro-optimizations: The skipping of
topology updates after updating disabled or unregistered regions.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>