Reset task state 'migrating' on nova compute restart

This change has the compute manager's init_instance method check
for a task state set to 'migrating' and resets the task state to
None, so that live migrations that were pending when the compute
service went down do not result in instances that are perpetually
stuck in 'migrating' task state.

Change-Id: If23ea983cc048f317daae594f420a643f1bfe6ac
Closes-Bug: #1339823
Enabled qemu memory balloon stats

We can get vm memory stats from libvirt API
'virDomainMemoryStats', it help telemetry module
like as: Ceilometer to collect vm memory usage,
but by default the memory statistical feature is
disable in qemu, we need to add stats period
in order to enabled memory statistical.

DocImpact: Adds mem_stats_period_seconds to
the [libvirt] group of nova.conf

Change-Id: Ie5c615565ffbe5d70394f2ddba162ace14abe24e
Implements: blueprint enabled-qemu-memballoon-stats
warn against sorting requirements

Change-Id: I64ae9191863564e278a35d42ec9cd743a233028e
Closes-Bug: 1365061
Merge "Reset task state 'migrating' on nova compute restart"
Merge "warn against sorting requirements"
Merge "Enabled qemu memory balloon stats"