qemu: properly set MaxMemLock when hotplugging with VFIO

qemu: properly set MaxMemLock when hotplugging with VFIO

This resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=1035490

virProcessSetMaxMemLock() (which is a wrapper over prlimit(3)) expects
the memory size in bytes, but libvirt's domain definition (which was
being used by qemuDomainAttachHostPciDevice()) stores all memory
tuning parameters in KiB. This was being accounted for when setting
MaxMemLock at domain startup time (so cold-plugged devices would
work), but not for hotplug.

This patch simplifies the few lines that call
virProcessSetMemMaxLock(), and multiply the amount * 1024 so that
we're locking the correct amount of memory.

What remains a mystery to me is why hot-plug of a managed='no' device
would succeed (at least on my system) while managed='yes' would
fail. I guess in one case the memory was coincidentally already
resident and in the other it wasn't.