KVM guest: KVM Steal time registration

KVM guest: KVM Steal time registration

This patch implements the kvm bits of the steal time infrastructure.
The most important part of it, is the steal time clock. It is an
continuous clock that shows the accumulated amount of steal time
since vcpu creation. It is supposed to survive cpu offlining/onlining.

[marcelo: fix build with CONFIG_KVM_GUEST=n]

Signed-off-by: Glauber Costa <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Tested-by: Eric B Munson <[email protected]>
CC: Jeremy Fitzhardinge <[email protected]>
CC: Peter Zijlstra <[email protected]>
CC: Avi Kivity <[email protected]>
CC: Anthony Liguori <[email protected]>
Signed-off-by: Avi Kivity <[email protected]>
Signed-off-by: Marcelo Tosatti <[email protected]>