xen/vm-event: Remove unnecessary vm_event_domain indirection

xen/vm-event: Remove unnecessary vm_event_domain indirection

The use of (*ved)-> leads to poor code generation, as the compiler can't
assume the pointer hasn't changed, and results in hard-to-follow code.

For both vm_event_{en,dis}able(), rename the ved parameter to p_ved, and
work primarily with a local ved pointer.

This has a key advantage in vm_event_enable(), in that the partially
constructed vm_event_domain only becomes globally visible once it is
fully constructed.  As a consequence, the spinlock doesn't need holding.

Furthermore, rearrange the order of operations to be more sensible.
Check for repeated enables and an bad HVM_PARAM before allocating
memory, and gather the trivial setup into one place, dropping the
redundant zeroing.

No practical change that callers will notice.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>