qemu: remove pointless qemuDomainLogContextMode

qemu: remove pointless qemuDomainLogContextMode

Since its introduction in 4d1b771fbb610537b7425e649a490143588b8ed3
it has only been used to differentiate between START and non-START.

Last use of QEMU_DOMAIN_LOG_CONTEXT_MODE_ATTACH was removed by:

  commit f709377301b919a9fcbfc366e33057f7848bee28
    qemu: Fix qemuDomainObjTaint with virtlogd

QEMU_DOMAIN_LOG_CONTEXT_MODE_STOP is unused since:

  commit cf3ea0769c54a328733bcb0cd27f546e70090c89
    qemu: process: Append the "shutting down" message using the new APIs

Now, the only caller passes QEMU_DOMAIN_LOG_CONTEXT_MODE_START.
Assume that's always the case and remove the 'mode' argument.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>