Input: xen-kbdfront - add grant reference for shared page

Without a grant reference, full access to the domain's memory is
required to use the shared page. Add an additional parameter in
xenstore to allow grant mapping to be used.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>