Properly report an error instead of panicing when user tries to create
LUN backed by non-disk device, e.g. /dev/null.

Reviewed by:	ken (earlier version)
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Workaround lldb issue with main module base address

On FreeBSD lldb sometimes reloads the the main module's (executable's)
symbols at the wrong address.  Work around this for now by explicitly
reloading at base_address=0 when it happens.

A proper fix is needed but early testers have reported this issue so
this workaround should allow them to make further progress.

http://llvm.org/bugs/show_bug.cgi?id=17880