xl: allow nameless domains to be named

xl: allow nameless domains to be named

At present, find_domain() will exit(2) if you specify a domain by
number, but that domain doesn't have a corresponding name.  However,
nothing seem to critically depend on common_domname being set, and the
test prevents dom0 or other nameless domains from being named.  So
just remove the check.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Yang Hongyang <yanghy@cn.fujitsu.com>