perf kvm: reuse some code of perf_kvm__timerfd_create()

Create a new function perf_kvm__timerfd_set() to reuse some code
in perf_kvm__timerfd_create()

Signed-off-by: Runzhen Wang <runzhen@linux.vnet.ibm.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
perf kvm: move the prompt_integer() to /util/top.c

As David suggested, this patch has no functional changes
just a code move.

move prompt_integer() from tools/perf/builtin-top.c  into
tools/perf/util/top.c

Signed-off-by: Runzhen Wang <runzhen@linux.vnet.ibm.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
perf kvm: set live mode refresh time dynamically

David's patch (https://lkml.org/lkml/2013/5/9/125 ) add a live mode for
perf kvm stat, but his patch can't set the refresh frequency
dynamically.

This patch accepts user's input and changes the value of
kvm->display_time variable.

For example, when the live mode is running, you press any key, it will
prints:

Mapped keys:
     [d]     set refresh frequency
     [q]     quit

Enter selection, or unmapped key to continue:
Enter refresh frequency(seconds): 2

Then, the live mode will refresh the screen every 2 seconds.

Signed-off-by: Runzhen Wang <runzhen@linux.vnet.ibm.com>
Signed-off-by: David Ahern <dsahern@gmail.com>