lib/posix-event: Update `uio_offset` and `uio_resid` on successful reads

lib/posix-event: Update `uio_offset` and `uio_resid` on successful reads

This commit updates (on a sucessful read) the `uio_offset` and
`uio_resid` members of the `uio` structure passed to the
`eventfd_vfscore_read` function. Without updating those members, the
`read` function from the vfscore layer will always report that no bytes
have been read at all.

Signed-off-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@unikraft.io>
Reviewed-by: Delia Pavel <delia_maria.pavel@stud.acs.upb.ro>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #798