xen/vm-event: Expand vm_event_* spinlock macros and rename the lock

xen/vm-event: Expand vm_event_* spinlock macros and rename the lock

These serve no purpose, but to add to the congnitive load of following
the code.  Remove the level of indirection.

Furthermore, the lock protects all data in vm_event_domain, making
ring_lock a poor choice of name.

For vm_event_get_response() and vm_event_grab_slot(), fold the exit
paths to have a single unlock, as the compiler can't make this
optimisation itself.

No functional change.

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