kvm: Clean up signal handling

kvm: Clean up signal handling

KVM used to use two signals, one for instruction count exits and one
for timer exits. There is really no need to distinguish between the
two since they only trigger exits from KVM. This changeset unifies and
renames the signals and adds a method, kick(), that can be used to
raise the control signal in the vCPU thread. It also removes the early
timer warning since we do not normally see if the signal was
delivered.

--HG--
extra : rebase_source : cd0e45ca90894c3d6f6aa115b9b06a1d8f0fda4d