Once QEMU is started, the qemuDomainLogContext is owned by it, and can
no longer be used from libvirt. Instead, use
qemuDomainLogAppendMessage() which will redirect the log.

This is not strictly necessary for swtpm, but the following patches
are going to reuse qemuExtDeviceLogCommand().
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>