xend: hot-plug PCI devices at boot-time

xend: hot-plug PCI devices at boot-time

Currently there are two interfaces to pass-through PCI devices:
1. A method driven through per-device xenstore entries that is used at
boot-time
2. An event-based method used for hot-plug.

This seems somewhat redundant and makes extending the code cumbersome
and prone to error - often the change needs to be made twice, in
two different ways.

This patch unifies PCI pass-through by using the existing event-based
method at boot-time.

Signed-off-by: Simon Horman <horms@verge.net.au>