x86/oprofile: Modify initialization of num_virt_counters

Models that do not yet support counter multiplexing have to setup
num_virt_counters. This patch implements the setup from num_counters
if num_virt_counters is not set. Thus, num_virt_counters must be setup
only for multiplexing support.

Signed-off-by: Robert Richter <robert.richter@amd.com>