vhost-user: Don't pass file descriptor for VHOST_USER_REM_MEM_REG

vhost-user: Don't pass file descriptor for VHOST_USER_REM_MEM_REG

The spec clarifies now that QEMU should not send a file descriptor in a
request to remove a memory region. Change it accordingly.

For libvhost-user, this is a bug fix that makes it compatible with
rust-vmm's implementation that doesn't send a file descriptor. Keep
accepting, but ignoring a file descriptor for compatibility with older
QEMU versions.

Signed-off-by: Kevin Wolf <[email protected]>
Message-Id: <20220407133657[email protected]>
Reviewed-by: Raphael Norwitz <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>