This partly reverts df90ca76.

Don't disable the VirtualBox driver when configure can't find
VBoxXPCOMC.so, rely on detection at runtime again instead.

Keep --with-vbox=/path/to/virtualbox intact, added to for:
https://bugzilla.redhat.com/show_bug.cgi?id=609185

Detection order for VBoxXPCOMC.so:

1. VBOX_APP_HOME environment variable
2. configure provided location
3. hardcoded list of known locations
4. dynamic linker search path

Also cleanup the glue code and improve error reporting.