system call time increase when turning on CONFIG_PARAVIRT

[Tim Chen <tim.c.chen@linux.intel.com>]

With CONFIG_PARAVIRT turned on, I've found that time invoking
system_call jumped up quite a lot.  Using TCP streaming test as a
workload and running on 32-bit 2.6.20 kernel, system_call goes up from
0.00025% all the way to 1.6% in the oprofile data.  There is a drop of
about 4% in overall throughput for this particular workload. 

With lmbench's null system call test, the call time goes up from 0.10
usec to 0.225 usec.

I'm testing on dual socket Intel core 2 processor running at 2.67 GHz
with 4 GB RAM.

Tim