resize: Specify format of input, output disk (RHBZ#642934,CVE-2010-3851).

resize: Specify format of input, output disk (RHBZ#642934,CVE-2010-3851).

This adds two new options: --format specifies the format of the
input disk, and --output-format specified the format of the output
disk.

Requiring the format of the output disk seems a bit strange at first:
after all, this is the disk that the virt-resize user has to create.
However it is needed because we sometimes reopen this disk, after
copying data over the first sector, and in theory a raw-format guest
could write a qcow2 header here and have it copied to the output
disk, which we would subsequently reopen.