virt-df: Match output with native (coreutils) 'df' command (RHBZ#578123).

virt-df: Match output with native (coreutils) 'df' command (RHBZ#578123).

This commit fixes the 'Use%' field in the output so it matches what
coreutils' 'df' command would print.

Firstly we change the calculation to use the space available to root,
not the space available to non-root.  This means it matches what 'df'
when run as root in the guest would show.

Secondly we display this rounded up to the next whole percent (ie. using
ceil), which is also what 'df' does.

Thirdly we change the regression test so it tests this.

Note that even with these changes you are not guaranteed to get precisely
the same figures from inside and outside the guest, as it depends on
how quiescent the guest is and how recently the superblock was synced.

Thanks: Rita Wu