qemu: Generate agent socket path if missing

qemu: Generate agent socket path if missing

It's not desired to force users imagine path for a socket they
are not even supposed to connect to. On the other hand, we
already have a release where the qemu agent socket path is
exposed to XML, so we cannot silently drop it from there.
The new path is generated in form:

$LOCALSTATEDIR/lib/libvirt/qemu/channel/target/$domain.$name

for qemu system mode, and

$XDG_CONFIG_HOME/qemu/lib/channel/target/$domain.$name

for qemu session mode.