cpu: fix typo: rename __kvm_hv_spinlock to __kvm_hv_spinlocks

cpu: fix typo: rename __kvm_hv_spinlock to __kvm_hv_spinlocks

Strings associated with virDomainHyperv values in domain_conf.c are used to
construct HyperV CPU features names to be compared with names defined in
cpu_x86_data.h and the names for HyperV "spinlocks" feature don't match.
This leads to a misleading warning:
"host doesn't support hyperv 'spinlocks' feature" even when it's supported.
Let's fix it and rename along with it VIR_CPU_x86_KVM_HV_SPINLOCK to
VIR_CPU_x86_KVM_HV_SPINLOCKS.

Signed-off-by: Maxim Nestratov <mnestratov@virtuozzo.com>