kvm: Silence warning from valgrind

valgrind complains here about uninitialized bytes with the following message:

==17814== Syscall param ioctl(generic) points to uninitialised byte(s)
==17814==    at 0x466A780: ioctl (in /usr/lib64/power8/libc-2.17.so)
==17814==    by 0x100735B7: kvm_vm_ioctl (kvm-all.c:1920)
==17814==    by 0x10074583: kvm_set_ioeventfd_mmio (kvm-all.c:574)

Let's fix it by using a proper struct initializer in kvm_set_ioeventfd_mmio().

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>