xen/arm: Correctly implement domain_page_map_to_mfn

xen/arm: Correctly implement domain_page_map_to_mfn

The function domain_page_map_to_mfn can be used to translate a virtual
address mapped by both map_domain_page and map_domain_page_global.
The former is using vmap to map the mfn, therefore domain_page_map_to_mfn
will always fail because the address is not in DOMHEAP range.

Check if the address is in vmap range and use __pa to translate it.

This patch fix guest shutdown when the event fifo is used.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>