torture: Remove whitespace from identify_qemu_vcpus output

The identify_qemu_vcpus bash function can return numbers including
whitespace characters, which can be a bit annoying in some bash
dollar-sign substitutions.  This commit therefore strips all spaces and
tabs from the value that identify_qemu_vcpus outputs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>