x86/hyperv: pass hv_vpindex cpu flag along with hv_synic

Since Qemu commit 9b4cf107b09d ("hyperv: only add SynIC in compatible
 configurations") hv_vpindex flag is mandatory if hv_synic is used:

$ qemu-system-x86_64 -nodefaults -cpu kvm64,hv_time,hv_synic,hv_stimer  \
     -device hyperv-testdev -device pc-testdev -device isa-debug-exit,  \
     iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev \
     -machine accel=kvm -kernel x86/hyperv_stimer.flat

Results in:

Hyper-V SynIC (requested by 'hv-synic' cpu flag) requires Hyper-V VP_INDEX
 ('hv-vpindex')
qemu-system-x86_64: kvm_init_vcpu failed: Function not implemented

Fix the configuration.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>