tools/libx[cl]: Plumb 'missing' through static_data_done() up into libxl

tools/libx[cl]: Plumb 'missing' through static_data_done() up into libxl

Pre Xen-4.14 streams will not contain any CPUID/MSR information.  There is
nothing libxc can do about this, and will have to rely on the higher level
toolstack to provide backwards compatibility.

To facilitate this, extend the static_data_done() callback, highlighting the
missing information, and modify libxl to use it.  At the libxc level, this
requires an arch-specific hook which, for now, always reports CPUID and MSR as
missing.  This will be adjusted in a later change.

No overall functional change - this is just plumbing.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>