sysbus: Add dynamic sysbus device search

sysbus: Add dynamic sysbus device search

Sysbus devices can be spawned by C code or dynamically via the command line.
In the latter case, we need to be able to find the dynamically created devices
to do things with them.

This patch adds a search helper that makes it easy to look for dynamically
spawned sysbus devices.

Signed-off-by: Alexander Graf <agraf@suse.de>