virsh-domain-monitor: add human readable output for 'domblkinfo'.

virsh-domain-monitor: add human readable output for 'domblkinfo'.

https://bugzilla.redhat.com/show_bug.cgi?id=1330940

The virsh command 'domblkinfo' returns the capacity, allocation and phisycal
size of the devices attached in a domain. Usually, this sizes are very big
and hard to understand and calculate. This commits introduce a human readable
support to check the size of each field easilly.

For example, the command before:

virsh # domblkinfo my_domain hda
Capacity:       21474836480
Allocation:     14875545600
Physical:       21474836480

and after this patch:

virsh # domblkinfo my_domain hda --human
Capacity:       20.000G
Allocation:     13.900G
Physical:       20.000G

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>