Add check for binary existing in machine type probe

Add check for binary existing in machine type probe

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