sw_inventory: consolidate os_probe logic

Reuse os_probe() function from probe.py and change the fallback
case to "unknown" to prevent OS id mismatch in ogserver.

The OS id mismatch causes the images to stop being associated with
partitions after the /refresh command.

src/utils/probe.py

@@ -177,4 +177,4 @@ def os_probe(mountpoint):
     elif os.path.exists(winreghives):
         return getwindowsversion(winreghives)
     else:
-        return ''
+        return 'unknown'

src/utils/sw_inventory.py

@@ -15,7 +15,7 @@ from collections import namedtuple
 
 import hivex
 
-from src.utils.probe import getwindowsversion, getlinuxversion
+from src.utils.probe import os_probe
 
 
 Package = namedtuple('Package', ['name', 'version'])
@@ -119,17 +119,13 @@ def _get_package_set_dpkg(dpkg_status_path):
 
 def get_package_set(mountpoint):
     dpkg_status_path = f'{mountpoint}{DPKG_STATUS_PATH}'
-    winreghives = f'{mountpoint}{WINDOWS_HIVES_PATH}'
-    osrelease = f'{mountpoint}{OSRELEASE_PATH}'
     softwarehive = f'{mountpoint}{WINDOWS_HIVES_SOFTWARE}'
     if os.path.exists(softwarehive):
         pkgset = _get_package_set_windows(softwarehive)
-        osname = getwindowsversion(winreghives)
     elif os.path.exists(dpkg_status_path):
         pkgset = _get_package_set_dpkg(dpkg_status_path)
-        osname = getlinuxversion(osrelease)
     else:
         pkgset = set()
-        osname = "unknown"
+    osname = os_probe(mountpoint)
     # Legacy software inventory first element is the OS name
     return [osname] + list(pkgset)