loader.efi: efipart should exclude iPXE stub block protocol

iPXE does insert stub BLOCK IO protocol handle to rework other issues,
this handle is not usable as it does not provide actual implementation.

We can detect this situation by checking and validating the BlockSize
property, so this update does make sure we have BlockSize at least 512B
and its value is power of 2.

PR: 223969
Reported by: Jeff Pieper
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D13297