x86/hvm: serialize trap injecting producer and consumer

x86/hvm: serialize trap injecting producer and consumer

Since injection works on a remote vCPU, and since there's no
enforcement of the subject vCPU being paused, there's a potential race
between the producing and consuming sides. Fix this by leveraging the
vector field as synchronization variable.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
[re-based]
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>