In which qmp-shell will exclusively use the HMP passthrough feature,
this is useful for testing.

Example:

    # ./qmp-shell -H qmp-sock
    Welcome to the HMP shell!
    Connected to QEMU 0.13.50

    (QEMU) info network
    VLAN 0 devices:
      user.0: net=10.0.2.0, restricted=n
        e1000.0: model=e1000,macaddr=52:54:00:12:34:56
        Devices not on any VLAN:
    (QEMU)

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>