dump: Improve error message when target doesn't support memory dump

The QERR_ macros are leftovers from the days of "rich" error objects.
We've been trying to reduce their remaining use.

Get rid of a use of QERR_UNSUPPORTED, and improve the rather vague
error message

    (qemu) dump-guest-memory mumble
    Error: this feature or command is not currently supported

to

    Error: dumping guest memory is not supported on this target

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-3-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
[Error message tweaked]