do-partial-upgrade crashed with SIGSEGV under wayland

[Impact]

Users can't upgrade to next Ubuntu release running the upgrade GUI under Wayland if a full upgrade fails and update-manager fails back to a partial upgrade.
The backport is needed to let Artful users to upgrade to Bionic without applying workarounds manually.

[Test Case (for update-manager)]

1. Install 17.10 and all the updates available
2. TODO: Make changes to make the full upgrade fail and as a result update-manager fall back to partial upgrade.
3. Start a wayland session (Ubuntu in GDM)
4. Check that root is not listed in xhost output:
  $ xhost
  access control enabled, only authorized clients can connect
  SI:localuser:rbalint
5. If root is present, remove it:
  $ xhost -si:localuser:root
  localuser:root being removed from access control list
  $ xhost
  access control enabled, only authorized clients can connect
  SI:localuser:rbalint
6. open a terminal and run the following command:
  $ update-manager -cd
7. When you're prompted to upgrade to 18.04, accept and proceed.
  Expected result on fixed version: Upgrade starts.
  Result on unfixed version: The GUI crashes.

[Test Case (for ubuntu-release-upgrader)]

1. Install 17.10 and all the updates available
2. Start a wayland session (Ubuntu in GDM)
3. Check that root is not listed in xhost output:
  $ xhost
  access control enabled, only authorized clients can connect
  SI:localuser:rbalint
4. If root is present, remove it:
  $ xhost -si:localuser:root
  localuser:root being removed from access control list
  $ xhost
  access control enabled, only authorized clients can connect
  SI:localuser:rbalint
5. open a terminal and run the following command:
  $ /usr/lib/ubuntu-release-upgrader/do-partial-upgrade --frontend=DistUpgradeViewGtk3
6. When you're prompted to upgrade to 18.04, accept and proceed.
  Expected result on fixed version: Upgrade asks for password to raise privileges then starts.
  Result on unfixed version: The command crashes.

[Test Case (for regressions)]

Repeat the test cases running under X.org (Ubuntu on Xorg in GDM) and check that:
a. Upgrade still works
b. xhost output is not modified

[Regression Potential]

The upgrade GUI may crash in a different way when applying the workaround fails.
Do-partial-upgrade may fail to detect development or versions to upgrade to.
The workaround "xhost +si:localuser:root" may be added under Xorg as well

[Other Info]
This bug covers performing a a partial upgrade which case is not covered int the fix for LP: #1732185 .