KVM: x86: Zero out PV features cache when the CPUID leaf is not present

KVM: x86: Zero out PV features cache when the CPUID leaf is not present

Clear KVM's PV feature cache prior when processing a new guest CPUID so
that KVM doesn't keep a stale cache entry if userspace does KVM_SET_CPUID2
multiple times, once with a PV features entry, and a second time without.

Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
Cc: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-19-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>