xend: Remember bootable flag for vbds in xenstore

xend: Remember bootable flag for vbds in xenstore

When xend is restarted, bootable flags of all disk devices are lost
and then the first disk is marked as bootable by a "compatibility
hack". When a guest domain is created with a mixture of several vbd
and tap devices, the compatibility hack may fail to choose the right
bootable device. Thus preventing the guest to be restarted. This patch
fixes this behavior by remembering bootable flag for each disk device
in xenstore database.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>