fix(gstfsd): find the guest OS by inspection, answer once, and stop a change at its deadline

gstfsd mounted every partition it listed: a root on LVM was never found,
a disk with two OS roots got the change twice and two concatenated
replies, and no root, an error or a request longer than 1024 bytes ended
the connection without a reply. It now lets libguestfs inspect the guest,
mounts its one OS the way the guest does (a broken filesystem off the
changed path is left out), and sends exactly one reply, keeping the first
error. The root entry is found on any line of /etc/shadow and must be
unique; a key needs no shadow file. A request is at most 64 KiB. Each
change runs in a worker process with a directly started appliance; both
are killed when the request is not done 100 s after gstfsd accepted it
(GSTFSD_DEADLINE may only lower that), before the panel's 120 s timeout
releases the VM, or when the panel's connection ends, and the reply waits
until they are gone, so nothing touches the disk once the panel has given
up. A worker that dies says why. The panel
connects over IPv4 or IPv6, sends the whole request and reads the reply
until gstfsd closes.

README.md

@@ -523,6 +523,7 @@ A rollback leaves the clone on a detached HEAD at the restored commit; the updat
 - "VM Clone Auto Migrate" is removed; a clone stays on its source host.
 - A template VM is changed only by superusers and staff owners with change permission; other owners can still view, open the console and clone it.
 - The log rotates (10 MB × 5) in `data/webvirtcloud.log`.
+- gstfsd finds the guest's operating system the way libguestfs inspects it, so a root on LVM works, and it always answers (an error when it finds no or several operating systems). Update `/usr/local/bin/gstfsd` on every compute (see [Adding a Compute Node](#adding-a-compute-node), step 3).
 - Docker no longer publishes port 6080; the console goes through nginx on the panel's port (`WS_PUBLIC_PORT`, default 80).
 
 ## Usage