Instead of having a separate function, we can simply return
zero from the existing qemuDomainGetMemLockLimitBytes() to
signal the caller that the memory locking limit doesn't need
to be set for the guest.

Having a single function instead of two makes it less likely
that we will use the wrong value, which is exactly what
happened when we started applying the limit that was meant
for VFIO-using guests to <memoryBacking><locked>-using
guests.