Add basic infrastructure to run tasks on vCPUs

This patch adds kvm_cpu__run_on_all_cpus() to run a task on each vCPU.
This infrastructure uses signals to signal the vCPU to allow a task
to be added to each vCPU's task. The vCPU executes any pending tasks
in the cpu run loop

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Will Deacon <will.deacon@arm.com>