vDPA/ifcvf: implement device MSIX vector allocator

This commit implements a MSIX vector allocation helper
for vqs and config interrupts.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Link: https://lore.kernel.org/r/20220222115428.998334-4-lingshan.zhu@intel.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>