perf kvm: Introduce option -v for perf kvm command.

As there is no -v option for perf kvm, the all debug message for perf
kvm will nerver be printed out to user.

Example:
	# perf kvm --guestmount /tmp/guestmount/ record -a
	Not enough memory for reading perf file header

It is confusing message for newbies such as me. With this patch applied,
we can use -v option to get the detail.

Example:
	# perf kvm --guestmount /tmp/guestmount/ record -a -v
	Can't access file /tmp/guestmount//15069/proc/kallsyms
	Not enough memory for reading perf file header

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1386609311-23889-1-git-send-email-yangds.fnst@cn.fujitsu.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>