x86: Clean up the hypervisor layer

Clean up the hypervisor layer and the hypervisor drivers, using an ops
structure instead of an enumeration with if statements.

The identity of the hypervisor, if needed, can be tested by testing
the pointer value in x86_hyper.

The MS-HyperV private state is moved into a normal global variable
(it's per-system state, not per-CPU state).  Being a normal bss
variable, it will be left at all zero on non-HyperV platforms, and so
can generally be tested for HyperV-specific features without
additional qualification.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Greg KH <greg@kroah.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Ky Srinivasan <ksrinivasan@novell.com>
LKML-Reference: <4BE49778.6060800@zytor.com>