Generic partition creation interface.

Generic partition creation interface.

This commit introduces a generic partition creation interface
which should be future-proof and extensible, and partially
replaces the old sfdisk-based interface.

The implementation is based on parted but is hopefully not too
dependent on the particulars of parted.

The following new calls are introduced:

  guestfs_part_init:
    Initialize a disk with a partition table.  Unlike the sfdisk-
    based interface, we also support GPT and other partition
    types, which is essential to scale to devices larger than 2TB.

  guestfs_part_add: Add a partition to an existing disk.

  guestfs_part_disk:
    Convenience function which combines part_init & part_add,
    creating a single partition that covers the whole disk.

  guestfs_part_set_bootable:
  guestfs_part_set_name:
    Set various aspects of existing partitions.

  guestfs_part_list:
    List partitions on a device.  This returns a programming-friendly
    list of partition structs (in contrast to sfdisk-l which cannot
    be parsed).

  guestfs_part_get_parttype:
    Return the partition table type, eg. "msdos" or "gpt".

The following calls are planned, but not added currently:

  guestfs_part_get_bootable
  guestfs_part_get_name
  guestfs_part_set_type
  guestfs_part_get_type