qemu-iotests: move check-block back to Makefiles

qemu-iotests: move check-block back to Makefiles

check-block has its own test harness, unlike every other test.  If
we capture its output, as is in general nicer to do without V=1,
there will be no sign of progress.  So for lack of a better option
just move the invocation of the test back to Makefile rules.

As a side effect, this will also fix "make check" in --disable-tools
builds, as they were trying to run qemu-iotests without having
made qemu-img before.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>