qemu: domain: Add helper for generating 'fdset' ids for VM startup

qemu: domain: Add helper for generating 'fdset' ids for VM startup

When starting a VM we must assign unique IDs for fdsets we add via
'-add-fd'. For now it was done by using the index of the filedescriptor
passed to the virCommand. That approach is not very flexible, because
you need to have already passed the 'fd' to virCommand before generating
the fdset path, and also won't nicely work with fdsets containing two or
more fds.

This patch introduces a counter into the private data of a qemu domain
so that we can allocate unique ids without relying on virCommand.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>