vircgroupv2devices: fix counting entries in BPF map

vircgroupv2devices: fix counting entries in BPF map

BPF syscall BPF_MAP_GET_NEXT_KEY returns -1 if something fails but it
will also return -1 if trying to get next key using the last key in the
map with errno set to ENOENT.

If there are VMs running and libvirtd is restarted and user tries to
call some cgroup devices operation on a VM we need to get the count of
entries in BPF map and it fails which will result in error when trying
to attach/detech devices.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1833321

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>