v2v: Add notes about layout of Export Storage Domain for multi-disk guests.
mllib: Add 'combine3' function.
v2v: -o rhev, -o vdsm: Fix export with multiple disks (RHBZ#1150475).

When translating the original virt-v2v code, I misunderstood how image
directories are created.  There is one directory under images/ per
disk image, not one for all the disks in a single guest.

Note this requires that -o vdsm uses multiple --vdsm-image-uuid
options, one per disk image of the guest.

Reported by: Alain Vondra
Thanks: Tingting Zheng