virLXCProcessReadLogOutputData: Refill buffer after filtering out noise

virLXCProcessReadLogOutputData: Refill buffer after filtering out noise

The caller passes in a 1k buffer, which when debug logging is in use is
easily filled with debug messages only. Thus after the first pass which
is common if the controller process already terminated the buffer will
not contain the real error, but rather a truncated debug message,
which will result in an error such as:

  error: internal error: guest failed to start: 2023-08-01 12:58:31.948+0000: 798195: i

instead of the proper error:

 error: internal error: guest failed to start: Failure in libvirt_lxc startup: Failed to create /home/rootfs/.oldroot: Permission denied

To fix the above retry the reading loop if the filtering function made
space in the buffer.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>