tools/kvm_stat: add command line switch '-c' to log in csv format

Add an alternative format that can be more easily used for further
processing later on.
Note that we add a timestamp in the first column for both, the regular
and the new csv format.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Message-Id: <20200306114250.57585-5-raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>