qemu_domain_address.c: turn qemuDomainFillDeviceIsolationGroup to void

qemu_domain_address.c: turn qemuDomainFillDeviceIsolationGroup to void

Starting on commit 1f43393283ff, qemuDomainFillDeviceIsolationGroup()
returns 0 in all circunstances. Let's turn it to 'void' make it
clearer that the function will not fail. This also spares a
check for < 0 return in qemu_hotplug.c. The
qemuDomainFillDeviceIsolationGroupIter() callback now returns
0 at all times - which is already happening anyway.

Refer to 1f43393283ff commit message for more details on why
the function was changed to never return an error.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>