util: Introduce qemu_get_pid_name

This is a small helper that tries to fetch binary name for given
PID.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <4d75d475c1884f8e94ee8b1e57273ddf3ed68bf7.1474987617.git.mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>