Check that blkfront devices have a non-zero number of sectors and a
non-zero sector size. Such a device would be a virtual disk of zero
bytes; clearly not useful, and not something we should try to attach.

As a fortuitous side effect, checking that these values are non-zero
here results in them not *becoming* zero later on the function. This
odd behaviour began with r309124 (clang 3.9.0) but is challenging to
debug; making any changes to this function whatsoever seems to affect
the llvm optimizer behaviour enough to make the unexpected zeroing of
the sector_size variable cease.

PR: 215209
Security: The potential for variables to unexpectedly become zero

		has worrying consequences for security in general, but
		not so much in this particular context.