Convert userspace_io_test to use vm_create_with_one_vcpu() and pass around
a 'struct kvm_vcpu' object instead of using a global VCPU_ID.  Note,
this is a "functional" change in the sense that the test now creates a vCPU
with vcpu_id==0 instead of vcpu_id==1.  The non-zero VCPU_ID was 100%
arbitrary and added little to no validation coverage.  If testing non-zero
vCPU IDs is desirable for generic tests, that can be done in the future by
tweaking the VM creation helpers.

Opportunistically use vcpu_run() instead of _vcpu_run() with an open
coded assert that KVM_RUN succeeded.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>