i386: Omit all-zeroes entries from KVM CPUID table

i386: Omit all-zeroes entries from KVM CPUID table

KVM has a 80-entry limit at KVM_SET_CPUID2.  With the
introduction of CPUID[0x1F], it is now possible to hit this limit
with unusual CPU configurations, e.g.:

  $ ./x86_64-softmmu/qemu-system-x86_64 \
    -smp 1,dies=2,maxcpus=2 \
    -cpu EPYC,check=off,enforce=off \
    -machine accel=kvm
  qemu-system-x86_64: kvm_init_vcpu failed: Argument list too long

This happens because QEMU adds a lot of all-zeroes CPUID entries
for unused CPUID leaves.  In the example above, we end up
creating 48 all-zeroes CPUID entries.

KVM already returns all-zeroes when emulating the CPUID
instruction if an entry is missing, so the all-zeroes entries are
redundant.  Skip those entries.  This reduces the CPUID table
size by half while keeping CPUID output unchanged.

Reported-by: Yumei Huang <yuhuang@redhat.com>
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1741508
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190822225210.32541-1-ehabkost@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>