tools/kvm_stat: fix debugfs handling

Te checks for debugfs assumed that debugfs is always mounted at
/sys/kernel/debug - which is likely, but not guaranteed. This is addressed
by checking /proc/mounts for the actual location.
Furthermore, when debugfs was mounted, but the kvm module not loaded, a
misleading error pointing towards debugfs not present was given.
To reproduce,
(a) run kvm_stat with debugfs mounted at a place different from
    /sys/kernel/debug
(b) run kvm_stat with debugfs mounted but kvm module not loaded

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>