KVM: Prevent overflow in KVM_GET_SUPPORTED_CPUID

The number of entries is multiplied by the entry size, which can
overflow on 32-bit hosts.  Bound the entry count instead.

Reported-by: David Wagner <daw@cs.berkeley.edu>
Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>