Clean-up code of one-letter identifiers

[dataclouder]

The whole code base is littered with identifiers (variables, struct types, and types) made of a single letter.

To make the library code more readable, and easier to maintain and expand, these identifiers must be replaced with more telling names.

The only place where single letter variables can be accepted is for a counter in a short loop.

Moreover, variables and field name vdc and vcd should be avoided as they are easily mixed up, and replaced with vcDirector and vdCenter or similarly telling names.

[dataclouder]

Comment from @lvirbalas:

vCD and vDC are such core abbreviations in vCloud Director, that people just get used to the one
letter difference between the two. Same in code, for a person who is working with vCD for years
(and that is our customers), "vcd" and "vdc" are less confusing than the suggested "vcdir" and
"vdcent". BTW, a common longer abbreviation in the wide open internet for "vcd" is "vcloud".
As there's potentially more harm than benefit of changing these abbreviations, let's keep using "vcd" and "vdc".