xen,kdump: handle pv domain in paddr_vmcoreinfo_note()

For kdump to work correctly it needs the physical address of
vmcoreinfo_note. When running as dom0 this means the virtual address
has to be translated to the related machine address.

paddr_vmcoreinfo_note() is meant to do the translation via
__pa_symbol() only, but being attributed "weak" it can be replaced
easily in Xen case.

Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Petr Tesarik <ptesarik@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>