qemu-char: qemu_chr_open_fd() can't fail, don't check

Cleaned up silently in commit aad04cd0, but that just got reverted.
Re-apply this part.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
