KVM: x86: Encapsulate CPUID entries and metadata in struct

Add a struct to hold the array of CPUID entries and its associated
metadata when handling KVM_GET_SUPPORTED_CPUID.  Lookup and provide
the correct entry in do_host_cpuid(), which eliminates the majority of
array indexing shenanigans, e.g. entries[i -1], and generally makes the
code more readable.  The last array indexing holdout is kvm_get_cpuid(),
which can't really be avoided without throwing the baby out with the
bathwater.

No functional change intended.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>