x86/hvm: Fix altp2m_vcpu_enable_notify error handling

x86/hvm: Fix altp2m_vcpu_enable_notify error handling

The altp2m_vcpu_enable_notify subop handler might skip calling
rcu_unlock_domain() after rcu_lock_current_domain().  Albeit since both
rcu functions are no-ops when run on the current domain, this doesn't
really have repercussions.

The second change is adding a missing break that would have potentially
enabled #VE for the current domain even if it had intended to enable it
for another one (not a supported functionality).

Signed-off-by: Adrian Pop <apop@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>