spapr: Capabilities infrastructure
    
    Because PAPR is a paravirtual environment access to certain CPU (or other)
    facilities can be blocked by the hypervisor.  PAPR provides ways to
    advertise in the device tree whether or not those features are available to
    the guest.
    
    In some places we automatically determine whether to make a feature
    available based on whether our host can support it, in most cases this is
    based on limitations in the available KVM implementation.
    
    Although we correctly advertise this to the guest, it means that host
    factors might make changes to the guest visible environment which is bad:
    as well as generaly reducing reproducibility, it means that a migration
    between different host environments can easily go bad.
    
    We've mostly gotten away with it because the environments considered mature
    enough to be well supported (basically, KVM on POWER8) have had consistent
    feature availability.  But, it's still not right and some limitations on
    POWER9 is going to make it more of an issue in future.
    
    This introduces an infrastructure for defining "sPAPR capabilities".  These
    are set by default based on the machine version, masked by the capabilities
    of the chosen cpu, but can be overriden with machine properties.
    
    The intention is at reset time we verify that the requested capabilities
    can be supported on the host (considering TCG, KVM and/or host cpu
    limitations).  If not we simply fail, rather than silently modifying the
    advertised featureset to the guest.
    
    This does mean that certain configurations that "worked" may now fail, but
    such configurations were already more subtly broken.
    
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    Reviewed-by: Greg Kurz <groug@kaod.org>
    (cherry picked from commit 33face6b8981add8eba1f7cdaf4cf6cede415d2e)
     Conflicts:
    	include/hw/ppc/spapr.h
    *drop context dep on 60c6823b9bc
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>