libxl: Set def->vcpus after successfully modifying live vcpu count

libxl: Set def->vcpus after successfully modifying live vcpu count

def->vcpus was never updated after successfully changing the live
vcpu count of a domain. Subsequent queries for vcpu info would
return incorrect results.  E.g.:

virsh vcpucount test
maximum      config         4
maximum      live           4
current      config         4
current      live           4

virsh setvcpus test 2

virsh vcpucount test
maximum      config         4
maximum      live           4
current      config         4
current      live           4

After patch, live current config is reported correctly:

virsh vcpucount test
maximum      config         4
maximum      live           4
current      config         4
current      live           2

While fixing this, noticed that the live config was not saved
to cfg->stateDir via virDomainSaveStatus. Save the live config
and change error handling of virDomainSave{Config,Status} to
log a message via VIR_WARN, instead of failing the entire
DomainSetVcpusFlags operation.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>