qga/commands-posix: Rework build_guest_fsinfo_for_real_device() function

We are going to support non-PCI devices soon. For this we need to split
the generic GuestDiskAddress and GuestDiskAddressList memory allocation
and list chaining into a separate function first.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>