hostmem-file: reject invalid pmem file sizes

Guests started with NVDIMMs larger than the underlying host file produce
confusing errors inside the guest.  This happens because the guest
accesses pages beyond the end of the file.

Check the pmem file size on startup and print a clear error message if
the size is invalid.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1669053


Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Zhang Yi <yi.z.zhang@linux.intel.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190214031004.32522-3-stefanha@redhat.com>
Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Pankaj Gupta <pagupta@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>