staging: unisys: Add visor device find routine

If we are going to remove the bus_info structs than we need a way
to find the devices when the *_create/destroy cmds are sent over
the vmchannel.

This function crudely impements what pci has.  It takes a bus_no
and dev_no and finds the matching 'struct visor_device'.

This function can/should be optimzed later once we get our heads
wrapped around its needs.  For now, I am using dev_no=0 to mean
the visorbus itself.

The function is limited to chipset.c only because it is only needed
to do the lookups upon receiving a vmchannel command.  Future patches
will make sure the resulting 'struct visor_device' is used every
where else.

Also allow visorbus_type to be more visible for use.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>