kvm__setup_cpuid: use KVM_GET_SUPPORTED_CPUID to simplify cpuid setup

kvm__setup_cpuid: use KVM_GET_SUPPORTED_CPUID to simplify cpuid setup

Let kernel setup "struct kvm_cpuid2" for us!

https://kerneltrap.org/mailarchive/linux-kernel/2010/4/27/4563640
Avi Kivity:
'''
No.  KVM_GET_CPUID2 reads what was set by KVM_SET_CPUID, as modified by
the guest executing the cpuid instruction.  KVM_GET_SUPPORTED_CPUID
tells userspace which bits are supported by the host cpu and kvm.

The API for KVM_GET_SUPPORTED_CPUID returns all cpuid leaves supported
in one go, IIRC.
'''
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>