formatdomain.html.in: document emulator/vcpu pin delay

In a guest with only one vcpu, when pinning the emulator in say CPU184
and the vcpu0 in CPU0 of the host, the user might expect that only
CPU0 and CPU184 of the host will be used by the guest.

The reality is that Libvirt takes some time to honor the emulator
and vcpu pinning, taking care of NUMA constraints first. This will
result in other CPUs of the host being potentially used by the
QEMU thread until the emulator/vcpu pinning is done. The user
then might be confused by the output of 'virsh cpu-stats' in this
scenario, showing around 200 microseconds of cycles being spent
in other CPUs.

Let's document this behavior, which is explained in detail in
Libvirt commit v5.0.0-199-gf136b83139, in the cputune section
of formatdomain.html.in.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>