qemu: convert monitor to use qemuDomainLogContextPtr indirectly

qemu: convert monitor to use qemuDomainLogContextPtr indirectly

Currently the QEMU monitor is given an FD to the logfile. This
won't work in the future with virtlogd, so it needs to use the
qemuDomainLogContextPtr instead, but it shouldn't directly
access that object either. So define a callback that the
monitor can use for reporting errors from the log file.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>