Speed up loading block drivers when running with --hostonly

Speed up loading block drivers when running with --hostonly

Instead of grovelling through all the modules available for the
kernel looking for block devices, only look at the modules that are
actually loaded. This speeds things up by a rather large amount
when generating the initramfs with --hostonly.

While we are at it, only load the filesystem module that will actually
be used for the root filesystem when running in --hostonly instead
of all the filesystem modules that happen to be loaded at the time.