xen/grant: Switch common/grant_table.c to use typesafe MFN

xen/grant: Switch common/grant_table.c to use typesafe MFN

At the same time replace MFN 0 by INVALID_MFN or drop the initializer
when it is not necessary. This will make clearer that the MFN
initialized is not valid.

Other than MFN 0 -> INVALID_MFN, no functional change intended.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>