xen/acpi: allow xen-acpi-processor driver to load on Xen 4.7

xen/acpi: allow xen-acpi-processor driver to load on Xen 4.7

As of Xen 4.7 PV CPUID doesn't expose either of CPUID[1].ECX[7] and
CPUID[0x80000007].EDX[7] anymore, causing the driver to fail to load on
both Intel and AMD systems. Doing any kind of hardware capability
checks in the driver as a prerequisite was wrong anyway: With the
hypervisor being in charge, all such checking should be done by it. If
ACPI data gets uploaded despite some missing capability, the hypervisor
is free to ignore part or all of that data.

Ditch the entire check_prereq() function, and do the only valid check
(xen_initial_domain()) in the caller in its place.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit 6f2d9d99213514360034c6d52d2c3919290b3504)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>