lib/ukfile: Add `uk_iov_remaining` to help tell remaining iov space

lib/ukfile: Add `uk_iov_remaining` to help tell remaining iov space

Add a new routine `uk_iov_remaining` that simply checks how many bytes
the memory regions described by `iov[iovcnt]`, starting at `cur` offset
from the buffer at `iov[iovi]` can fit.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1619