Add a mutex to guarantee single writer to qemu_logfile handle.

Add a mutex to guarantee single writer to qemu_logfile handle.

Also added qemu_logfile_init() for initializing the logfile mutex.

Note that inside qemu_set_log() we needed to add a pair of
qemu_mutex_unlock() calls in order to avoid a double lock in
qemu_log_close().  This unavoidable temporary ugliness will be
cleaned up in a later patch in this series.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20191118211528.3221-4-robert.foley@linaro.org>