qemu: domain: Store and update 'fdsetindex' across libvirtd restarts

qemu: domain: Store and update 'fdsetindex' across libvirtd restarts

While 'add-fd' qmp command gives the possibility to find an unused fdset
ID when hot-adding fdsets, such usage is extremely inconvenient.

This patch allows us to track the used fdset id so that we can avoid the
need to check results and thus employ simpler code flow when hot-adding
devices which use FD passing.

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