Some HP BIOSes (observed with a Z840) seem to attempt to connect our
drivers in the middle of our call to DisconnectController().  The
precise chain of events is unclear, but the symptom is that we see
several calls to our Supported() and Start() methods, followed by a
system lock-up.

Work around this dubious BIOS behaviour by explicitly failing calls to
our Start() method while we are in the middle of attempting to
disconnect drivers.

Reported-by: Jordan Wright <jordan.m.wright@disney.com>
Debugged-by: Adrian Lucrèce Céleste <adrianlucrececeleste@airmail.cc>
Debugged-by: Christian Nilsson <nikize@gmail.com>
Tested-by: Jordan Wright <jordan.m.wright@disney.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>