qga-win: Handle fstrim for OSes lower than Win8

qga-win: Handle fstrim for OSes lower than Win8

The defrag.exe tool which is used for executing the fstrim command
on Windows doesn't support retrim for OSes lower than Win8. This
commit handles this case and returns a suitable error.

Output of fstrim before this commit:
{"execute":"guest-fstrim"}
{"return": {"paths": [{"path": "C:\\", "error": "An invalid command line option
was specified. (0x89000008)"}, {"path": "F:\\", "error": "An invalid command
line option was specified. (0x89000008)"}, {"path": "S:\\", "error": "An
invalid command line option was specified. (0x89000008)"}]}}

Reported on:
https://bugzilla.redhat.com/show_bug.cgi?id=1594113

Signed-off-by: Sameeh Jubran <sjubran@redhat.com>
* use alternative version query code proposed by Sameeh
* fix up version check logic
* avoid CamelCase variable names when possible
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>