util/log: re-allow switching away from stderr log file

util/log: re-allow switching away from stderr log file

Commit 59bde21374 ("util/log: do not close and reopen log files when
flags are turned off") prevented switching away from stderr on a
subsequent invocation of qemu_set_log_internal(). This prevented
switching away from stderr with the 'logfile' monitor command as well
as an invocation like
> ./qemu-system-x86_64 -trace 'qemu_mutex_lock,file=log'
from opening the specified log file.

Fixes: 59bde21374 ("util/log: do not close and reopen log files when flags are turned off")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20231004124446.491481-1-f.ebner@proxmox.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit f05142d511e86d8e97967d21f205d990dfc634de)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>