x86: kvmclock: allocate pvclock shared memory area

We want to expose the pvclock shared memory areas, which
the hypervisor periodically updates, to userspace.

For a linear mapping from userspace, it is necessary that
entire page sized regions are used for array of pvclock
structures.

There is no such guarantee with per cpu areas, therefore move
to memblock_alloc based allocation.

Acked-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>