Add API error DUPLICATE_MAC_SEED

Signed-off-by: John Else <john.else@citrix.com>
Add message Pool.assert_mac_seeds_available

Signed-off-by: John Else <john.else@citrix.com>
CA-101238:Check for duplicate mac_seeds during VM.create

Signed-off-by: Akshay Ramani <akshay.ramani@citrix.com>
Merge pull request #1171 from akshayramani/CA-101238.a

Ca 101238: XOP-321 xapi doesn't check for duplicate MAC seeds during VM.create