Removed SD card still be displayed

[Evan Wang]

$ system-image-cli -i
current build number: 309
device name: krillin
channel: ubuntu-touch/rc-proposed/bq-aquaris.en
last update: 2016-04-11 11:05:52
version version: 309
version ubuntu: 20160408.3
version device: 20160329-a9bacdb
version custom: 20160324--36-54-vivid

Steps to reproduce:
1. Booting device with a SD card inserted
2. Make sure the contents can be seen on the SD card in Nautilus
3. Remove the SD card
4. Insert another SD card
5. Check the Nautilus window and try to browse the contents on each SD card

Actual results:
There are two SD cards' icon shown in the Nautilus window, moreover the contents can be seen on the two SD cards (one has two music files under Music directory, another one is empty under Music directory).

Repeated sdcard entries listed per inode after insert/remove the sdcards for many times.

$ ls -l /var/run/user/$(id -u)/gvfs/mtp:*
total 0
drwx------ 1 evan evan 0 Jan 1 1970 Aquaris E4.5 Ubuntu Edition
drwx------ 1 evan evan 0 Jan 1 1970 C54D-25D1
drwx------ 1 evan evan 0 Jan 1 1970 C54D-25D1
drwx------ 1 evan evan 0 Jan 1 1970 C54D-25D1
drwx------ 1 evan evan 0 Jan 1 1970 D340-A54A
drwx------ 1 evan evan 0 Jan 1 1970 D340-A54A