cpu: Reclaim vCPU objects

cpu: Reclaim vCPU objects

In order to deal well with the kvm vcpus (which can not be removed without any
protection), we do not close KVM vcpu fd, just record and mark it as stopped
into a list, so that we can reuse it for the appending cpu hot-add request if
possible. It is also the approach that kvm guys suggested:
https://www.mail-archive.com/[email protected]/msg102839.html

Signed-off-by: Chen Fan <[email protected]>
Signed-off-by: Gu Zheng <[email protected]>
Signed-off-by: Zhu Guihua <[email protected]>
Signed-off-by: Bharata B Rao <[email protected]>
               [- Explicit CPU_REMOVE() from qemu_kvm/tcg_destroy_vcpu()
                  isn't needed as it is done from cpu_exec_exit()
                - Use iothread mutex instead of global mutex during
                  destroy
                - Don't cleanup vCPU object from vCPU thread context
                  but leave it to the callers (device_add/device_del)]
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Signed-off-by: David Gibson <[email protected]>