OvmfPkg/XenBusDxe: Fix some types.

This patch replace some types in GrantTable and the argument Index of
XenHypercallHvmGetParam to what the types should be.

This avoid to have type cast in code.

Contributed-under: TianoCore Contribution Agreement 1.0

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Build-tested-by: Scott Duplichan <scott@notabs.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16391 6f19259b-4bc3-4df7-8a09-765794883524