vmdk: make vmdk_is_cid_valid a coroutine_fn

vmdk: make vmdk_is_cid_valid a coroutine_fn

Functions that can do I/O are prime candidates for being coroutine_fns.  Make the
change for the one that is itself called only from coroutine_fns.  Unfortunately
vmdk does not use a coroutine_fn for the bulk of the open (like qcow2 does) so
vmdk_read_cid cannot have the same treatment.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230309084456.304669-10-pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>