esx: Fix MAC address formatting

esx: Fix MAC address formatting

VMware uses two MAC address prefixes: 00:0c:29 and 00:50:56. The 00:0c:29
prefix is used for ESX server generated addresses. The 00:50:56 prefix is
split into two parts. MAC addresses above 00:50:56:3f:ff:ff are generated
by a vCenter. The rest of the 00:50:56 prefix can be assigned manually.
Any MAC address within the 00:0c:29 and 00:50:56 prefix can be specified
in a domain XML config and the driver will handle the details internally.

* src/esx/esx_vmx.c: fix MAC address formatting
* tests/xml2vmxdata/*: update test files accordingly