kvm-ifup: Use /bin/bash

kvm-ifup: Use /bin/bash

As reported by Ben Beuchler, kvm-ifup uses bash-specific constructs and
doesn't work with Ubuntu's default shell. We use bash all over the place
anyway, so changing the shebang line is the easiest solution.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>