x86/hvm_event: fix uninitialized struct field usage introduced by c/s f5365e6

x86/hvm_event: fix uninitialized struct field usage introduced by c/s f5365e6

c/s f5365e6: "xen/vm-events: Move parts of monitor_domctl code to common-side",
introduced a use without initialization issue.
hvm_event_breakpoint calls hvm_event_traps(&req) and if sync is true that
ors some bits into req->flags which was never initialised.
Reported by Coverity Scan.

Initializes req @ hvm_event_breakpoint entry.

Coverity-ID: 1353192
Signed-off-by: Corneliu ZUZU <czuzu@bitdefender.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>