dump: Fix HMP dump-guest-memory -z without -R

dump: Fix HMP dump-guest-memory -z without -R

-z without -R has no effect: the dump format remains @elf.  Fix the
logic error so it becomes @kdump-zlib.

Fixes: e6549197f7ed (dump: Add command interface for kdump-raw formats)
Fixes: CID 1523841
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>