GPT add-on doesn't write protective MBR on GPT init.

[kallisti5]

While GPT now works, it doesn't write out the protective MBR. Not a *huge* issue, but something we should investigate long-term.

GPT disk created under Haiku, edited in gdisk:

$ gdisk test-gpt.img 
GPT fdisk (gdisk) version 1.0.0

Partition table scan:
  MBR: not present
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with corrupt MBR; using GPT and will write new
protective MBR on save.

Command (? for help): p
Disk test-gpt.img: 2097152 sectors, 1024.0 MiB
Logical sector size: 512 bytes
Disk identifier (GUID): 230DDDCA-B80E-EA4F-90FD-DF416876B88B
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 2097118
Partitions will be aligned on 8-sector boundaries
Total free space is 1981 sectors (990.5 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          327719   160.0 MiB   EB00  
   2          327720         1036327   346.0 MiB   EB00  
   3         1036328         2095143   517.0 MiB   EB00  

Command (? for help): q

[kallisti5]

confirming still an issue as of hrev50543

[jessicah]

Fixed in hrev50722.