Allow the user to request only a specific subset of statistics.
This can be useful when working on a feature or optimization that is
known to affect that statistic.

Example:

   (qemu) info stats vcpu halt_poll_fail_ns
   provider: kvm
       halt_poll_fail_ns (cumulative, ns): 0

In case multiple providers have the same statistic, the provider can be
specified too:

   (qemu) info stats vcpu halt_poll_fail_ns kvm
   provider: kvm
       halt_poll_fail_ns (cumulative, ns): 0

Extracted from a patch by Mark Kanda.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>