build: fix "make check" without earlier "make"

"make check", if not preceded by "make", will not build the tools
needed by qemu-iotests.  This happens because qemu-iotests, aka
"make check-block", is not yet part of meson.build.

While at it, remove the reference to the now-dead QEMU_IOTESTS_HELPERS-y
variable.

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