Config option to enable sync writes during block migration

There is a new feature introduced in Libvirt 8.0.0 that allows block migrations to converge even under high IO.

Non-shared storage migration of guests which are disk I/O intensive and have fast local storage may actually never converge if the guest happens to dirty the disk faster
than it can be copied.

This patch introduces a new flag 'VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES' which will instruct hypervisors to synchronize local I/O writes with the writes to remote storage used for migration so that the guest can't overwhelm the migration. This comes at a cost of decreased local I/O performance for guests which behave well on average.

New config option [libvirt]/live_migration_permit_sync_writes can be introduced to enable synchronous writes during block migrations