KVM: VMX: Return 0 from a failed VMREAD

If we execute VMREAD during reboot we'll just skip over it.  Instead of
returning garbage, return 0, which has a much smaller chance of confusing
the code.  Otherwise we risk a flood of debug printk()s which block the
reboot process if a serial console or netconsole is enabled.

Signed-off-by: Avi Kivity <avi@redhat.com>