vl.c: Exit QEMU early if no machine is found

We check whether the variable machine is NULL or not before accessing
it. If machine is NULL, exit QEMU with an error, this can avoids a
segfault error.

Markus Armbruster <armbru@redhat.com> adds that the segfault can be
reproduced as follows:

  $ qemu-system-xtensa -cpu help

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
