kvm: Fix uninitialized cpuid_data

This error was reported by valgrind when running qemu-system-x86_64
with kvm:

==975== Conditional jump or move depends on uninitialised value(s)
==975==    at 0x521C38: cpuid_find_entry (kvm.c:176)
==975==    by 0x5235BA: kvm_arch_init_vcpu (kvm.c:686)
==975==    by 0x4D5175: kvm_init_vcpu (kvm-all.c:267)
==975==    by 0x45035B: qemu_kvm_cpu_thread_fn (cpus.c:858)
==975==    by 0xD361E0D: start_thread (pthread_create.c:311)
==975==    by 0xD65E9EC: clone (clone.S:113)
==975==  Uninitialised value was created by a stack allocation
==975==    at 0x5226E4: kvm_arch_init_vcpu (kvm.c:446)

Instead of adding more memset calls for parts of cpuid_data, the existing
calls were removed and cpuid_data is now initialized completely in one
call.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
