UAPI: Put a comment into uapi/asm-generic/kvm_para.h and use it from arches

Make uapi/asm-generic/kvm_para.h non-empty by addition of a comment to stop
the patch program from deleting it when it creates it.

Then delete empty arch-specific uapi/asm/kvm_para.h files and tell the Kbuild
files to use the generic instead.

Should this perhaps instead be a #warning or #error that the facility is
unsupported on this arch?

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Arnd Bergmann <arnd@arndb.de>
cc: Avi Kivity <avi@redhat.com>
cc: Marcelo Tosatti <mtosatti@redhat.com>
cc: kvm@vger.kernel.org