tools/libxc: Calculate xstate cpuid leaf from guest information

tools/libxc: Calculate xstate cpuid leaf from guest information

The existing logic is broken for heterogeneous migration.  By always
advertising the host maximum xstate, a migration to a less capable host always
fails as Xen cannot accomodate the xcr0_accum in the migration stream.

By calculating xstate from the feature information (which a multi-host
toolstack will have levelled appropriately), the guest will have the current
hosts maximum xstate advertised, allowing for correct migration to less
capable hosts.

In addition, some further improvements and corrections:
 - don't discard the known flags in sub-leaves 2..63 ECX
 - zap sub-leaves beyond 62
 - zap all bits in leaf 1, EBX/ECX.  No XSS features are currently supported.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>