kvmtool: Fix regression introduced with d2a7ddff

Since commit d2a7ddff (Add minimal support for macvtap) opening
of tap device might fail. lkvm shows

  Warning: Config tap device error. Are you root?

virtio_net_request_tap passed wrong pointer for struct ifreq to
TUNSETIFF ioctl.

Signed-off-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>