libxl: Use vkb=[] for HVMs

libxl: Use vkb=[] for HVMs

xl/libxl only applies vkb=[] to PV & PVH guests.  HVM gets only a single
vkb by default, but that can be disabled by the vkb_device boolean.
Notably the HVM vkb cannot be configured, so feature-abs-pointer or the
backend-type cannot be specified.

Re-arrange the logic so that vkb=[] is handled regardless of domain
type.  If vkb is empty or unspecified, follow the vkb_device boolean for
HVMs.  Nothing changes for PVH & PV.  HVMs can now get a configured vkb
instead of just the default one.

The chance for regression is an HVM config with
vkb=["$something"]
vkb_device=false

Which would now get a vkb.

This is useful for vGlass which provides a VKB to HVMs.  vGlass wants to
specify feature-abs-pointer, but that is racily written by vGlass
instead of coming through the xl.cfg.  Unhelpfully, Linux xen-kbdfront
reads the backend nodes without checking that the backend is in
InitWait.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>