qga: guest-get-fsinfo: add optional 'total-bytes-privileged' field

qga: guest-get-fsinfo: add optional 'total-bytes-privileged' field

Since the commit 25b5ff1a86 ("qga: add mountpoint usage info to
GuestFilesystemInfo") we have 2 values reported in guest-get-fsinfo:
used = (f_blocks - f_bfree), total = (f_blocks - f_bfree + f_bavail) as
returned by statvfs(3).  While on Windows guests that's all we can get
with GetDiskFreeSpaceExA(), on POSIX guests we might also be interested in
total file system size, as it's visible for root user.  Let's add an
optional field 'total-bytes-privileged' to GuestFilesystemInfo struct,
which'd only be reported on POSIX and represent f_blocks value as returned
by statvfs(3).

While here, also tweak the docs to reflect better where those values
come from.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/r/20240320161648.158226-2-andrey.drobyshev@virtuozzo.com
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>