hv_xen: Report memory used by hypervisor
hv_xen: Report memory used by hypervisor

- Report memory used by hypervisor (“mem_hv” as per resource model
  design document, “xmem” in htools)
- Also report number of CPUs available to Dom0
- Some other, small changes

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: René Nussbaumer <bc755ac45a0e799d1d490cba76febc56d80cc5fc@google.com>

apyrgio/ganeti,bitemyapp/ganeti,bitemyapp/ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,apyrgio/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,ganeti/ganeti,leshchevds/ganeti,yiannist/ganeti,onponomarev/ganeti,mbakke/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,andir/ganeti,dimara/ganeti,grnet/snf-ganeti,andir/ganeti,andir/ganeti,yiannist/ganeti,leshchevds/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti