client.profilers.kvm_stat: Try to mount debugfs only when it is not mounted

Script will alway try to mount debugfs, if kvm_stat fail.

When "/usr/bin/kvm_stat --batch" fail, we may get following message:

Exit status: 1
Duration: 0.0273129940033

stdout:
Please mount debugfs ('mount -t debugfs debugfs /sys/kernel/debug')
and ensure the kvm modules are loaded

At this time debugfs may already mounted. But script will try to mount it again.

Signed-off-by: Feng Yang <fyang@redhat.com>