qemu: Keep QEMU host drive prefix in BlkIoTune

qemu: Keep QEMU host drive prefix in BlkIoTune

The QEMU -drive id= begins with libvirt's QEMU host drive prefix
("drive-"), which is stripped off in several places two convert between
host ("-drive") and guest ("-device") device names.

In the case of BlkIoTune it is unnecessary to strip the QEMU host drive
prefix because we operate on "info block"/"query-block" output that uses
host drive names.

Stripping the prefix incorrectly caused string comparisons to fail since
we were comparing the guest device name against the host device name.

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