KVM: selftests: Use the common cpuid() helper in cpu_vendor_string_is()

Use cpuid() to get CPUID.0x0 in cpu_vendor_string_is(), thus eliminating
the last open coded usage of CPUID (ignoring debug_regs.c, which emits
CPUID from the guest to trigger a VM-Exit and doesn't actually care about
the results of CPUID).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-42-seanjc@google.com