Comment 3 for bug 1307431

So the problem here, as is shown by your kernel log is that your container even though the rootfs upgraded properly is still using an old config...

That old config doesn't mount pstore which is then causing the hang at boot time which you are seeing.

LXC 1.0 fixes that issue going forward through a set of config includes, but for old containers, you may have to update their config by hand to be closer to what you'd get if you just created a new container.

So edit /var/lib/lxc/<container>/config and make it look like something like that:
"""
# Template used to create this container: /usr/share/lxc/templates/lxc-download
# Parameters passed to the template: -d ubuntu -r trusty -a amd64
# For additional config options, please look at lxc.conf(5)

# Distribution configuration
lxc.include = /usr/share/lxc/config/ubuntu.common.conf
lxc.arch = x86_64

# Container specific configuration
lxc.rootfs = /var/lib/lxc/YOUR-CONTAINER-NAME/rootfs
lxc.utsname = YOUR-CONTAINER-NAME

# Network configuration
lxc.network.type = veth
lxc.network.flags = up
lxc.network.link = lxcbr0
lxc.network.hwaddr = YOUR-CONTAINER-MAC-ADDRESS
"""

Then start it again and things should be back to normal.

There's sadly no way we can programmatically do that for existing containers on upgrade due to all the possible combinations and hand made changes, but as I said, the new lxc.includes we added should let us cover most of those cases moving forward and hopefully we won't have a similar problem with 16.04.