virBitmap: Place virBitmapIsAllClear check after virBitmapParse calls

virBitmap: Place virBitmapIsAllClear check after virBitmapParse calls

This patch adds checks for empty bitmaps right after the calls of
virBitmapParse. These only include spots where set API's are called and
where domain's XML is parsed.
Also, it partially reverts commit 983f5a which added a check for
invalid nodeset "0,^0" into virBitmapParse function. This change broke
the logic, as an empty bitmap should not cause an error.

https://bugzilla.redhat.com/show_bug.cgi?id=1210545