qemu_opts_parse(): always check return value

qemu_opts_parse() can always return NULL, even if the QemuOptsList.desc in
question would be trivial to satisfy (eg. because it's empty). For
example:

qemu_opts_parse()
  opts_parse()
    qemu_opts_create()
      id_wellformed()

In practice:

  $ .../qemu-system-x86_64 -acpitable id=3
  qemu-system-x86_64: -acpitable id=3: Parameter 'id' expects an identifier
  **
  ERROR:vl.c:3491:main: assertion failed: (opts != NULL)
  Aborted (core dumped)

  $ .../qemu-system-x86_64 -smbios id=3
  qemu-system-x86_64: -smbios id=3: Parameter 'id' expects an identifier
  Segmentation fault (core dumped)

I checked all qemu_opts_parse() invocations (and all drive_def()
invocations too, because it blindly forwards the former's retval). Only
the two above examples look problematic.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1385658779-7529-1-git-send-email-lersek@redhat.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>
