fix #2570: add 'keephugepages' config

fix #2570: add 'keephugepages' config

We already keep hugepages if they are created with the kernel
commandline (hugepagesz=x hugepages=y), but some setups (specifically
hugepages across multiple NUMA nodes) cannot be configured that way.
Since we always clear these hugepages at VM shutdown, rebooting a VM
that uses them might not work, since the requested count might not be
available anymore by the time we want to use them (also, we would then
no longer allocate them correctly on the NUMA nodes).

Add a 'keephugepages' parameter to skip cleanup and simply leave them
untouched.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>