tests: check path to avoid a failing qga/get-vcpus test

The qga/get-vcpus test fails in a simple chroot environment, as
used in an openSUSE Build Service local build, so first check
that the sysfs based path exists in order to avoid calling this
test in an environment where it won't work right.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
