tests: Request virtio-mmio for balloon-mmio-deflate

tests: Request virtio-mmio for balloon-mmio-deflate

For all supported QEMU version, the virt machine type has a hard
dependency on PCI support, so if we want to test virtio-balloon
together with virtio-mmio we have to either request that
explicitly or trick libvirt by masking capabilities. Do the
former instead of the latter.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>