blktrace: disallow -o when using multiple devices

blktrace: disallow -o when using multiple devices

Document that "-o" does not work when specyfing multiple devices to
blktrace, also: enforce this by stopping blktrace when one tries do
do this.

The technical reason why "-o" doesn't work with multiple devices is
because we use multiple threads of execution - one per device/CPU pair -
and each of them opens a file named "<prefix>.blktrace.<cpu>". With the
"-o" all of the "<prefix>" values are the same - so multiple threads
open the same file and try to do output. Not good. Without the "-o"
we get unique files named: "<device>.blktrace.<cpu>"  - as the tuple
(<device>,<cpu>) is unique.

Signed-off-by: Alan D. Brunelle <Alan.Brunelle@hp.com>