[efi] Allow creating an image device handle with no parent device

When we fall back to using our own loaded image's device handle
(instead of the most recently opened SNP device handle), we may find
that the device handle is no longer valid since we have disconnected
the driver that originally provided it.

Check for existence of the device path protocol on the identified
parent handle, and choose not to attempt to set a parent-child
relationship if the parent handle appears to no longer be valid.

Signed-off-by: Michael Brown <mcb30@ipxe.org>