iotests: Simplify qemu-iotests-quick.sh

As of the "iotests: Allow out-of-tree run" series, the qemu-iotests may
(and should) be run directly in the build tree and will then guess the
binary paths themselves. Therefore, qemu-iotests-quick.sh does not need
to (and should not) enter the source path anymore; also, it does not
need to specify the binaries because "check" will guess them
automatically.

As a side-effect, tests using qemu may now be added to the quick group.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
