ivshmem: use kvm irqfd for msi notifications

ivshmem: use kvm irqfd for msi notifications

Use irqfd for improving context switch when notifying the guest.
If the host doesn't support kvm irqfd, regular msi notifications are
still supported.

Note: the ivshmem implementation doesn't allow switching between MSI and
IO interrupts, this patch doesn't either.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>