Support for blocking FS

[CvvT]

Our current FS assumes every operation is nonblocking, which does not hold for 9p fs over network. To give the illusion that 9p fs is nonblocking, we use spin loop in the litebox_shim_linux that waits until an operation is done. This is a temporary workaround that makes it work for the SNP runner, in which case it might be fine because the remote 9p server supposes to run locally at VMPL0. We should not use it elsewhere.

Originally posted by @CvvT in #709