migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread

migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread

Added missing qemu_fflush() on buffer file holding precopy device state.
Increased initial QIOChannelBuffer allocation to 512KB to avoid reallocs.
Typical configurations often require >200KB for device state and VMDESC.

Fixes: 8518278a6af589ccc401f06e35f171b1e6fae800 (migration: implementation
  of background snapshot thread)
Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Message-Id: <20210401092226.102804-2-andrey.gruzdev@virtuozzo.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>