dfu: Make DFU virtual backend SPL friendly

Define stub for dfu_*_virt function in SPL, because
CONFIG_SPL_DFU_VIRT is not defined.

This patch avoids compilation issue in dfu_fill_entity() when
CONFIG_SPL_DFU is activated because the dfu_fill_entity_virt()
function is not available.

Fixes: ec44cace ("dfu: add DFU virtual backend")
Reported-by: Marek Vasut <marex@denx.de>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Marek Vasut <marex@denx.de>