Check if the 'start_bundle' function exists

The VIO6 hardware does not support the bundle mode.
Add a function to check if hardware supports the bundle mode

In fact, the VIO6 and VSP1 hardwares unsupport the bundle mode.
vio6: disable the register I/O output for debugging

The commit 'vio6: add 'shvio_blend' for blending (preliminary)'
(17e316d9bbe4cf3be9cd9170f772578c7bef1673) mistakenly added
the debug macro definition to enable log output of register I/Os.
This removes it.
vio6: keep a shortcut pointer to a sink entity

This may useful because some of functions must look for a sink entity
to access it.
vio6: fix hard-coded src/sink entities ID in vio6_set_src/src2/sink{_phys}

The src and sink entity IDs can be determined in each run since the
VIO hardware may have multiple RPFs and WPFs and the library can use
unoccupied ones on demand.
vio6: add emulation of the VEU bundle mode

This may be useful especially for applications which have been
implemented with using the VEU hardware.