x86/paravirt: Remove const mark from x86_hyper_xen_hvm variable

.. as "nopv" support needs it to be changeable at boot up stage.

Checkpatch reports warning, so move variable declarations from
hypervisor.c to hypervisor.h

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Juergen Gross <jgross@suse.com>