qemu: Fix qemuDomainObjTaint with virtlogd

qemu: Fix qemuDomainObjTaint with virtlogd

When virtlogd is used to capture QEMU's stdout, qemuDomainObjTaint would
always fail to write the message to the log file when QEMU is already
running (i.e., outside qemuProcessLaunch). This can happen during device
hotplug or by sending a custom QEMU guest agent command:

    warning : qemuDomainObjTaint:8757 : Domain id=9 name='blaf'
        uuid=9cfa4e37-2930-405b-bcb4-faac1829dad8 is tainted:
        custom-ga-command
    error : virLogHandlerDomainOpenLogFile:388 : Cannot open log file:
        '/var/log/libvirt/qemu/blaf.log': Device or resource busy
    error : virNetClientProgramDispatchError:172 : Cannot open log file:
        '/var/log/libvirt/qemu/blaf.log': Device or resource busy

The fix is easy, we just need to use the right API for appending a
message to QEMU log file instead of creating a new log context.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>