Move persistent_disk_in_use() from main

This patch creates a function persistent_disk_in_use() moving it's
content from main().

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>

tools/psdk

@@ -434,6 +434,24 @@ def scan_persistent_etc():
     for line in result.stdout.decode().splitlines():
         persistent_to_conf(line)
 
+def persistent_disk_in_use():
+    """Is the persistent disk in use?"""
+
+    persistent_disk_link = "/dev/disk/by-label/" + DISK_LABEL
+    if not os.path.islink(persistent_disk_link):
+        return False
+
+    persistent_disk = os.readlink(persistent_disk_link)
+    persistent_disk = "/dev/" + os.path.split(persistent_disk)[1]
+
+    command = ["sudo", "mount"]
+    result = _run(command)
+
+    if not persistent_disk in result.stdout.decode():
+        return False
+
+    return persistent_disk
+
 def main():
     """good old main"""
 
@@ -489,21 +507,12 @@ def main():
     else:
         # Configuration files management
 
-        persistent_disk_link = "/dev/disk/by-label/" + DISK_LABEL
-        if not os.path.islink(persistent_disk_link):
-            raise Exception("Persistent disk not initialized")
-
-        persistent_disk = os.readlink(persistent_disk_link)
-        persistent_disk = "/dev/" + os.path.split(persistent_disk)[1]
+        persistent_disk = persistent_disk_in_use()
+        if not persistent_disk:
+            raise Exception("Persistent disk not in use")
 
         disk = Disk(persistent_disk, DISK_LABEL)
 
-        command = ["sudo", "mount"]
-        result = _run(command)
-
-        if not persistent_disk in result.stdout.decode():
-            raise Exception("Persistent disk not currently in use.")
-
     if args.init:
         if fstab_updated():
             raise Exception("This SDK was already configured to use persistent workspace disk")