Fix interaction between CPU pinning and KVM migration

CPU pinning requires the KVM hypervisor to start in the paused state, in
order to retrieve information, and immediately unpauses it.

This does not play well with live migration, as the unpausing was done
before the migration started and so the receiving kvm process left the
migrated instance in the stopped status.

This patch fixes this behavior, by not launching the KVM process in
stopped state while on the receiving side of a migration.

Also, the stopping is now done outside _ExecuteCpuAffinity.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Tsachy Shacham <tsachy@google.com>