KVM: selftests: Rename 'struct vcpu' to 'struct kvm_vcpu'

Rename 'struct vcpu' to 'struct kvm_vcpu' to align with 'struct kvm_vm'
in the selftest, and to give readers a hint that the struct is specific
to KVM.

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