qga-win: fix installation on localized windows

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1357789
Replace hardcoded user and group names ("Administrators", "SYSTEM") with the ones acquired from system. Windows uses localized strings for these names and it may cause the installation to fail.
Windows has Well-known SIDs for "Administrators" group and "SYSTEM" user so they were used to identify required users and groups.
Well-known SIDs: https://support.microsoft.com/en-us/help/243330/well-known-security-identifiers-in-windows-operating-systems

Signed-off-by: Daniel Rempel <daniel@daynix.com>
Signed-off-by: Sameeh Jubran <sjubran@redhat.com>
Reviewed-by: Sameeh Jubran <sameeh@daynix.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
