keygen: Enhance read random loop

• Rewrite goto loop to while loop - this makes coverity error go away and increase readability
• Properly report used random file (there was hardcoded /dev/random)
• Test read returning 0 (end of file)
• Enhance message with bits needed (unsigned and size_t)

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>