x86: fix platform_export_bootpart() for Xen virtual disks

Virtual disk devices in a Xen virtual machine (DomU) can be /dev/xvda,
/dev/xvdb and so on with partitions like /dev/xdva1. Devices named like
this where not considered before. This resulted in a non working
sysupgrade, because the boot partition could not be found.

Signed-off-by: Alexander Dahl <alex@netz39.de>
Suggested-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 49190