The rewrite to use GLib's atomic ops functions changed the behavior
of virAtomicIntInc - before it returned the pre-increment value.

Most of the callers using its value were adjusted, but the one
in qemuDriverAllocateID was not. If libvirtd would reconnect to
a running domain during startup, the next started domain would get
the same ID:

$ virsh list
 Id   Name       State
--------------------------
 1    f28live    running
 1    f28live1   running

Use the g_atomic_add function directly (as recommended in viratomic.h)
and add 1 to the result.

This also restores the usual numbering from 1 instead of 0.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Fixes: 7b9645a7Reviewed-by: NPeter Krempa <pkrempa@redhat.com>