KVM: selftests: Convert xen_vmcall_test away from VCPU_ID

Convert xen_vmcall_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 make the "vm" variable local, it is unused outside of
main().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>