Add a new node filed x_mem
Add a new node filed x_mem

Nodes can have some memory unaccounted for, due to (e.g.) hypervisor
overhead, rounding errors in reporting, etc.

It is better if we model this memory explicitly instead of hiding it,
and actually since the n_mem addition it is actually required to do so.

The new attribute is not yet used.
yiannist/ganeti,mbakke/ganeti,leshchevds/ganeti,apyrgio/ganeti,onponomarev/ganeti,ganeti/ganeti,ganeti/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,bitemyapp/ganeti,onponomarev/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,yiannist/ganeti,andir/ganeti,dimara/ganeti,apyrgio/ganeti,dimara/ganeti,leshchevds/ganeti,ganeti/ganeti,grnet/snf-ganeti,mbakke/ganeti