pc: Fix machine property nvdimm-persistence error handling

pc: Fix machine property nvdimm-persistence error handling

Calling error_report() in a function that takes an Error ** argument
is suspicious.  pc.c's pc_machine_set_nvdimm_persistence() does that,
and then exit()s.  Wrong.  Attempting to set machine property
nvdimm-persistence to a bad value instantly kills the VM:

    $ qemu-system-x86_64 -nodefaults -S -display none -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 3}, "package": "v3.0.0-837-gc5e4e49258"}, "capabilities": []}}
    {"execute": "qmp_capabilities"}
    {"return": {}}
    {"execute": "qom-set", "arguments": {"path": "/machine", "property": "nvdimm-persistence", "value": "instadeath"}}
    -machine nvdimm-persistence=instadeath: unsupported option
    $ echo $?
    1

Broken when commit 11c39b5cd96 (v3.0.0) replaced error_propagate();
return by error_report(); exit() instead of error_setg(); return.  Fix
that.

Fixes: 11c39b5cd966ddc067a1ca0c5392ec9b666c45b7
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181017082702.5581-10-armbru@redhat.com>