qga-win: fix error-handling in getNameByStringSID()

In one case we misconstrue a BOOL return as an HRESULT, and in the
other case we don't check the BOOL return from LookupAccountSidW()
before extracting the HRESULT from GetLastError(). Both can lead to
getNameByStringSID() misreporting an error.

Reported-by: Chen Hanxiao <chenhanxiao@gmail.com>
Suggested-by: Tomáš Golembiovský <tgolembi@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
