qemu: Generate shorter channel target paths

qemu: Generate shorter channel target paths

A <channel/> device is basically an UNIX socket into guest.
Whatever is sent from the host, appears in the guest and vice
versa. But because of that, the length of the path to the socket
is important (underscored by fact that we derive the path from
domain short name). But there are still cases where we might not
fit into UNIX_PATH_MAX limit (usually 108 characters), because
the path is derived also from other variables, e.g.
XDG_CONFIG_HOME for session domains.

There are two components though, that are needless: "/target/"
and "domain-" prefix. Drop them. This is safe to do, because
running domains have their path saved in status XML and even
though paths are dropped on migration, they are not part of guest
ABI and thus we are free to change them.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>