Since commit cbe6d636 the command `qemu -accel help` returns
the list of accelerators enabled in the QEMU binary. This adds
the list_accel() method which return that same list.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20191216191438.93418-3-wainersm@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>