perf kvm: Support using -f to override perf.data.guest file ownership

perf kvm: Support using -f to override perf.data.guest file ownership

Enable perf kvm to use perf.data.guest when it is not owned by current
user or root.

Example:

 # perf kvm stat record ls
 # chown Yunlong.Song:Yunlong.Song perf.data.guest
 # ls -al perf.data.guest
 -rw------- 1 Yunlong.Song Yunlong.Song 4128937 Apr  2 11:05 perf.data.guest
 # id
 uid=0(root) gid=0(root) groups=0(root),64(pkcs11)

Before this patch:

 # perf kvm stat report
 File perf.data.guest not owned by current user or root (use -f to override)
 Initializing perf session failed
 # perf kvm stat report -f
   Error: unknown switch `f'

  usage: perf kvm stat report [<options>]

         --event <report event>
                           event for reporting: vmexit, mmio (x86 only),
                           ioport (x86 only)
         --vcpu <n>        vcpu id to report
     -k, --key <sort-key>  key for sorting: sample(sort by samples
 						   number) time (sort by avg time)
     -p, --pid <pid>       analyze events only for given process id(s)

As shown above, the -f option does not work at all.

After this patch:

 # perf kvm stat report
 File perf.data.guest not owned by current user or root (use -f to override)
 Initializing perf session failed
 # perf kvm stat report -f
 Analyze events for all VMs, all VCPUs:

   VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time

 Total Samples:0, Total events handled time:0.00us.

As shown above, the -f option really works now. Since we have not
launched any KVM related process, the result shows 0 sample here.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1427982439-27388-5-git-send-email-yunlong.song@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>