kvm: Support timing accesses for KVM cpu

kvm: Support timing accesses for KVM cpu
This patch enables timing accesses for KVM cpu.  A new state,
RunningMMIOPending, is added to indicate that there are outstanding timing
requests generated by KVM in the system.  KVM's tick() is disabled and the
simulation does not enter into KVM until all outstanding timing requests have
completed.  The main motivation for this is to allow KVM CPU to perform MMIO
in Ruby, since Ruby does not support atomic accesses.