ovs-vswitchd: Add --mlockall option and enable on XenServer.

ovs-vswitchd: Add --mlockall option and enable on XenServer.

On XenServer 5.5 we found that running 4 simultaneous vm-import operations
on iSCSI caused so much disk and cache activity that (we suspect) parts of
ovs-vswitchd were paged out to disk and were not paged back in for over
10 seconds, causing the XenServer to fall off the network and the XenCenter
connection to fail.

Locking ovs-vswitchd into memory appears to avoid this problem.  Henrik
reports that, with memory locking, importing 11 VMs simultaneously
completed successfully.

Bug #2344.