xl: Return an error if an empty file is passed to cd-insert

xl: Return an error if an empty file is passed to cd-insert

Two changes:
* Stat the file before calling libxl_cdrom_insert()
* Return an error if anything fails (including libxl_cdrom_insert)

This is in part to work around the fact that the RAW disk type
is used for things that aren't actually files; so we can't call
stat in libxl_device.c:libxl__device_disk_set_backend() because
it may be going over a remote protocol.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>