virtio: do not take address of packed members

The address of a packed member is not packed, which may cause accesses
to unaligned pointers.  Avoid this by reading the packed value before
passing it to another function.

Cc: Jason Wang <jasowang@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>