fix(cgpt): Change the hybrid MBR partition type to FAT32

Xen's pvgrub and grub 0.97 on which it is based do not recognize the EFI
partition type 0xef as a FAT filesystem. The "FAT32 LBA" type works.

For reference here is the list of types defined in grub 0.97:

    #define PC_SLICE_TYPE_HIDDEN_FLAG       0x10
    #define PC_SLICE_TYPE_NONE              0
    #define PC_SLICE_TYPE_FAT12             1
    #define PC_SLICE_TYPE_FAT16_LT32M       4
    #define PC_SLICE_TYPE_EXTENDED          5
    #define PC_SLICE_TYPE_FAT16_GT32M       6
    #define PC_SLICE_TYPE_FAT32             0xb
    #define PC_SLICE_TYPE_FAT32_LBA         0xc
    #define PC_SLICE_TYPE_FAT16_LBA         0xe
    #define PC_SLICE_TYPE_WIN95_EXTENDED    0xf
    #define PC_SLICE_TYPE_EZD               0x55
    #define PC_SLICE_TYPE_MINIX             0x80
    #define PC_SLICE_TYPE_LINUX_MINIX       0x81
    #define PC_SLICE_TYPE_EXT2FS            0x83
    #define PC_SLICE_TYPE_LINUX_EXTENDED    0x85
    #define PC_SLICE_TYPE_VSTAFS            0x9e
    #define PC_SLICE_TYPE_DELL_UTIL         0xde
    #define PC_SLICE_TYPE_LINUX_RAID        0xfd