hostmem-file: reject invalid pmem file sizes

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 <[email protected]>
Cc: Zhang Yi <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: Igor Mammedov <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20190214031004[email protected]>
Reviewed-by: Wei Yang <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Reviewed-by: Pankaj Gupta <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>