Convert evmcs_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==5.  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(), the test expects
KVM_RUN to succeed.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>