v2v: Check if there is sufficient free space for conversion (RHBZ#1139543).

Before trying to perform the conversion, estimate if there is
sufficient free space on the guest filesystem(s) for the conversion to
be successful.

If not, you will see an error similar to this one:

[   6.0] Checking for sufficient free disk space in the guest
virt-v2v: error: not enough free space for conversion on filesystem '/'.
4354048 bytes free < 20000000 bytes needed