qemu-iotests/199: prepare for new test-cases addition

Move future common part to start_postcopy() method. Move checking
number of bitmaps to check_bitmap().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Message-Id: <20200727194236.19551-19-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>