qemu.py: fix is_running() return before first launch()

is_running() returns None when called before the first time we
call launch():

    >>> import qemu
    >>> vm = qemu.QEMUMachine('qemu-system-x86_64')
    >>> vm.is_running()
    >>>

It should return False instead. This patch fixes that.

For consistence, this patch removes the parenthesis from the
second clause as it's not really needed.

Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20170901112829.2571-2-apahim@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>