dump: Drop qmp_dump_guest_memory() stub and build for all targets

dump: Drop qmp_dump_guest_memory() stub and build for all targets

qmp_dump_guest_memory() calls dump_init() and returns an Error when
cpu_get_dump_info() returns an error, as done by the stub.
So there is no need to have a stub for qmp_dump_guest_memory().

Enable the documentation of the always-present dump-guest-memory command.

That way we can drop CONFIG_HAVE_CORE_DUMP and leave configure
completely out of the picture for target CPU features.

Signed-off-by: Andreas Färber <afaerber@suse.de>