x86/vm_event: add support for VM_EVENT_REASON_INTERRUPT

x86/vm_event: add support for VM_EVENT_REASON_INTERRUPT

Added support for a new event type, VM_EVENT_REASON_INTERRUPT,
which is now fired in a one-shot manner when enabled via the new
VM_EVENT_FLAG_GET_NEXT_INTERRUPT vm_event response flag.
The patch also fixes the behaviour of the xc_hvm_inject_trap()
hypercall, which would lead to non-architectural interrupts
overwriting pending (specifically reinjected) architectural ones.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Acked-by: Julien Grall <julien.grall@arm.com>