This commit converts qemu_opts_create() from qerror_report() to
error_set().

Currently, most calls to qemu_opts_create() can't fail, so most
callers don't need any changes.

The two cases where code checks for qemu_opts_create() erros are:

 1. Initialization code in vl.c. All of them print their own
    error messages directly to stderr, no need to pass the Error
    object

 2. The functions opts_parse(), qemu_opts_from_qdict() and
    qemu_chr_parse_compat() make use of the error information and
    they can be called from HMP or QMP. In this case, to allow for
    incremental conversion, we propagate the error up using
    qerror_report_err(), which keeps the QError semantics

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-By: Laszlo Ersek <lersek@redhat.com>