This patch enables migrating vcpu's TSC rate. If KVM on the
destination machine supports TSC scaling, guest programs will
observe a consistent TSC rate across the migration.
If TSC scaling is not supported on the destination machine, the
migration will not be aborted and QEMU on the destination will
not set vcpu's TSC rate to the migrated value.
If vcpu's TSC rate specified by CPU option 'tsc-freq' on the
destination machine is inconsistent with the migrated TSC rate,
the migration will be aborted.
For backwards compatibility, the migration of vcpu's TSC rate is
disabled on pc-*-2.5 and older machine types.
Backports relevant parts of commit 36f96c4b6bd25f43000c317518ff3df10202bc75 from qemu