vm_event: Fix regression caused by b701ccc8

vm_event: Fix regression caused by b701ccc8

When xc_map_foreign_batch got deprecated reinitializing vm_event on a domain
where an event listener was previously active broke as it relied on the flag
XEN_DOMCTL_PFINFO_XTAB to indicate that the magic page is not in the physmap.
Manually check the gpfn type, add it to the physmap if needed, and only then
try to map it.

Signed-off-by: Tamas K Lengyel <tlengyel@novetta.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>