pci-assign: accept Error from monitor_handle_fd_param2()

pci-assign: accept Error from monitor_handle_fd_param2()

Propagate any errors in monitor fd handling up to get_real_device(), and
report them there. We'll continue the propagation upwards when
get_real_device() becomes a leaf itself (when none of its callees will
report errors internally any longer when detecting and returning an
error).

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
eviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>