target-i386: kvm: prevent buffer overflow if -cpu foo, [x]level is too big

Stack corruption may occur if too big 'level' or 'xlevel' values passed
on command line with KVM enabled, due to limited size of cpuid_data
in kvm_arch_init_vcpu().

reproduces with:
 qemu -enable-kvm -cpu qemu64,level=4294967295
or
 qemu -enable-kvm -cpu qemu64,xlevel=4294967295

Check if there is space in cpuid_data before passing it to cpu_x86_cpuid()
or abort() if there is not space.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
