Support generic disabling of all XSAVE features

[Andi Kleen]

For performance testing and debugging it can be useful to disable XSAVE
features individually. This patchkit hooks up XSAVE with the
generic clearcpuid=... option, so that disabling a CPUID feature
automatically disables the respective XSAVE feature.

It also cleans up CPUID dependency management. Currently it's
possible to generate configurations with clearcpuid that crash.

It replaces an earlier patchkit that did this with special
case options.

v1:
Initial post
v2:
Work around broken lguest by exporting set_cpu_cap
Repost with cover letter
v3:
Repost. No changes to code.