evtchn: slightly defer lock acquire where possible

evtchn: slightly defer lock acquire where possible

port_is_valid() and evtchn_from_port() are fine to use without holding
any locks. Accordingly acquire the per-domain lock slightly later in
evtchn_close() and evtchn_bind_vcpu(). Especially for the use by the
former (but there are pre-existing uses) add a comment about
port_is_valid()'s guarantees.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Julien Grall <julien@xen.org>