tools: Specify format of disks (RHBZ#642934,CVE-2010-3851).

tools: Specify format of disks (RHBZ#642934,CVE-2010-3851).

Sys::Guestfs::Lib is changed in two ways: firstly we take the format
string from libvirt and pass it to add_drive_opts.  Secondly we allow
an extra format =>  parameter to open_guest which allows the
format to be specified for disk images.

All the tools are changed to add an extra --format parameter allowing
the format to be specified for direct disk images.