virtio/fs/macos: keep a fd to unlinked files

On the macOS implementation of passthrough, we avoid keeping an
FD open for each dirent entry by accessing the inode using the
special directory '/.vol'. But this strategy doesn't work when
the inode has been unlinked, and it's completely valid for
userspace to keep accessing a file after unlinking it by keeping
an FD to it.

To fix this without having to keep an FD open as the Linux
implementation does (thus risking running out of open handles),
here we change do_unlink() to open an FD and store it in
InodeData->unlinked_fd. Then we change inode_to_path() to become
inode_to_handle(), being able to return either a path accssible
through '/.vol' or an FD. Finally, we update every user of
inode_to_handle() to be able to operate both with a path and with
an FD.

The FD stored in InodeData->unlinked_fd is released on
forget_one(), which is called when the guest removes the entry
from dirent.

Instead of wrapping the FD behind an OwnedFd or a File, we operate
on it as an integer/RawFd. This allows us to 1) store the value
as an atomic operation without a Mutex and 2) save a couple of
syscalls everytime we use it.

Signed-off-by: Sergio Lopez <slp@redhat.com>

Author: slp