qemu: Re-detect virtual cpu threads after cpu hot (un)plug.

qemu: Re-detect virtual cpu threads after cpu hot (un)plug.

After a cpu hotplug the qemu driver did not refresh information about
virtual processors used by qemu and their corresponding threads. This
patch forces a re-detection as is done on start of QEMU.

This ensures that correct information is reported by the
virDomainGetVcpus API and "virsh vcpuinfo".

A failure to obtain the thread<->vcpu mapping is treated non-fatal and
the mapping is not updated in a case of failure as not all versions of
QEMU report this in the info cpus command.