virsh: Add more human-friendly output of domblkstat command

virsh: Add more human-friendly output of domblkstat command

Users of virsh complain that output of the domblkstat command
is not intuitive enough. This patch adds explanation of fields
returned by this command to the help section for domblkstat and
the man page of virsh. Also a switch --human is added for
domblkstat that prints the fields with more descriptive
texts.

This patch also changes sequence of the output fields and their
names back to the order and spelling established by previous
versions of virsh to maintain compatibility with scripts.

Example of ordered and "translated" output:

PRE-patch:

virsh # domblkstat 1 vda
vda wr_bytes 5170176
vda wr_operations 511
vda rd_bytes 82815488
vda rd_operations 3726

POST-patch:

virsh # domblkstat 1 vda
vda rd_req 3726
vda rd_bytes 82815488
vda wr_req 478
vda wr_bytes 4965376

Example of human readable output:

virsh # domblkstat 1 vda --human
Device: vda
 number of read operations:      3726
 number of read bytes:           82815488
 number of write operations:     478
 number of bytes written:        4965376

https://bugzilla.redhat.com/show_bug.cgi?id=731656