qom: Fix regression with 'qom-type'

qom: Fix regression with 'qom-type'

Commit 9a6d1ac assumed that 'qom-type' could be removed from QemuOpts
with no ill effects.  However, this command line proves otherwise:

$ ./x86_64-softmmu/qemu-system-x86_64 -nodefaults -nographic -qmp stdio \
  -object rng-random,filename=/dev/urandom,id=rng0 \
  -device virtio-rng-pci,rng=rng0
qemu-system-x86_64: -object rng-random,filename=/dev/urandom,id=rng0: Parameter 'qom-type' is missing

Fix the regression by restoring qom-type in opts after its temporary
removal that was needed for the duration of user_creatable_add_opts().

Reported-by: Richard W. M. Jones <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Tested-by: Richard W.M. Jones <[email protected]>
Message-id: 20170323160315[email protected]
Signed-off-by: Peter Maydell <[email protected]>