[QMS-694] Porting Qt6: Fix decoding of device mount point

[kiozen]

What is the linked issue for this pull request:

QMS-#694

What you have done:

Refacture code to properly decode the DBus message

Steps to perform a simple smoke test:

Mount a Garmin device

Does the code comply to the coding rules and naming conventions Coding Guidelines:

• [x] yes

Is every user facing string in a tr() macro?

• [x] yes

Did you add the ticket number and title into the changelog? Keep the numeric order in each release block.

• [ ] yes, I didn't forget to change changelog.txt