When probing machine types if the QEMU binary does not exist
we get a hard to diagnose error, due to the execve() in the
child failing

error: internal error Child process exited with status 1.

Add an explicit check so that we get

error: Cannot find QEMU binary /usr/libexec/qem3u-kvm: No such file or directory

* src/qemu/qemu_capabilities.c: Check for QEMU binary