CP-9366: Watch the "control" xenstore node

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

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Signed-off-by: Rob Hoes <rob.hoes@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>
Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #113 from robhoes/hvmlinux

CP-9366: Use HVM feature-flags instead of checking PV drivers