kvm: Add kvm_set_user_memory tracepoint

This adds a tracepoint to trace the KVM_SET_USER_MEMORY_REGION ioctl
parameters which is quite useful for debugging VFIO memory regions
being actually registered with KVM.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20171215052326.21386-1-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>