virsh: Resolve Coverity NEGATIVE_RETURNS

virsh: Resolve Coverity NEGATIVE_RETURNS

Coverity notes that after we VIR_ALLOC_N(params, nparams) a failed call to
virDomainGetCPUStats could result in nparams being set to -1. In that case,
the subsequent virTypedParamsFree in cleanup will pass -1 which isn't good.

Use the returned value as the number of stats to display in the loop as
it will be the value reported from the hypervisor and may be less than
nparams which is OK

Signed-off-by: John Ferlan <jferlan@redhat.com>