api: document all fields returned by VolumeMeasureResult

[bennyz]

Currently only required is documented, but the rest of the available fields returned by it are:

{
    "required": 1073741824,
    "fully-allocated": 1073741824
    "bitmaps": 0
}

Engine needs bitmaps to measure correctly when copying volumes with bitmaps.
Note that bitmaps may be missing if there a no bitmaps in the qcow2 image
so the value should be optional (defaultvalue: null).

The bitmap field was added in 4.4.x - find the commit that started to report it,
and use the first release delivering this commit in the added property.

fully-allocated is not needed for anything, so better not document it.