analyze-migration.py doesn't account for saved blocks
Description of problem:

Steps to reproduce:
1. Make a migration snapshot that includes incremental block device (from HMP: `migrate -i "exec: cat > snap"`)
2. Load the snapshot: `scripts/analyze-migration.py -f snap` 


```
Traceback (most recent call last):
  File "scripts/analyze-migration.py", line 605, in <module>
    dump.read(dump_memory = args.memory)
  File "scripts/analyze-migration.py", line 539, in read
    classdesc = self.section_classes[section_key]
KeyError: ('block', 0)
```
Additional information:
Here's pseudocode derived from `block_load` in `migration/block.c`:

```
N blocks of the following:

  read 64 bits: sector number and flags
    (blk->sector << BDRV_SECTOR_BITS) | flags

  if flags & BLK_MIG_FLAG_EOS:
    break
  if flags & BLK_MIG_FLAG_PROGRESS
    continue
  if flags & BLK_MIG_FLAG_DEVICE_BLOCK
    byte: name length
    length bytes: device name string
    if not flags & BLK_MIG_FLAG_ZERO_BLOCK:
      read (1 << 20) bytes 
```