qemu-log: Open file for logging when specified

qemu-log: Open file for logging when specified

qemu-log defaults to stderr when there is no '-D' option mentioned on command
line. When '-D' option is specified, we also need to specify '-d' option for it
to use the specified logfile. When using monitor to enable logging this is
troublesome since there will be no '-d' option because of which monitor dumps
the logs to stderr.

Fix this by opening the log file when '-D' is specified on the command line.
Also fix an ancient comment which does not hold true since changing location and
log level has now been streamlined.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Luiz Capitulino <lcapitulino@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1433946024-18439-1-git-send-email-bobby.prani@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>