CP-9366: Watch the "control" xenstore node, and update VM-metrics "other".

Now when qemu writes a feature-flag into the control node, xapi will
notice and so will trigger the existing code that can copy those entries
into the "other" vm guest metrics parameter.

To make that work, when checking whether the guest metrics should really
be updated, now we go ahead even if only the "other" map has changed.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
CP-9366: Allow vCPU hotplug for HVM (if feature-vcpu-hotplug is set).

Removed code that raises an Unimplemented exception unconditionally
for hvm guests when attempting vCPU hotplug.
This allows the existing code that decides based on the feature-flag
to work properly.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>