qemu-option: Clean up fragile use of error_is_set()

qemu-option: Clean up fragile use of error_is_set()

Using error_is_set(ERRP) to find out whether to bail out due to
previous error is either wrong, fragile, or unnecessarily opaque.
It's wrong when ERRP may be null, because errors go undetected when it
is.  It's fragile when proving ERRP non-null involves a non-local
argument.  Else, it's unnecessarily opaque (see commit 84d18f0).

The error_is_set(state->errp) in qemu_opts_from_qdict_1() is merely
fragile, because the callers never pass state argument with null
state->errp.

Make the code more robust and more obviously correct: test
*state->errp directly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>