The MAC addresses with 00:50:56 prefix are split into several ranges:

  00:50:56:00:00:00 - 00:50:56:3f:ff:ff  'static' range (manually assigned)
  00:50:56:80:00:00 - 00:50:56:bf:ff:ff  'vpx' range (assigned by a VI Client)

Erroneously the 'vpx' range was assumed to be larger and to occupy the
remaining addresses of the 00:50:56 prefix that are not part of the 'static'
range.

00:50:56 was used as prefix for generated MAC addresses, this is not possible
anymore, because there are gaps in the allowed ranges. Therefore, change the
prefix to 00:0c:29 which is the prefix for auto generated MAC addresses anyway.

Allow arbitrary MAC addresses to be used and set the checkMACAddress VMX option
to false in case the MAC address doesn't fall into any predefined range.

* docs/drvesx.html.in: update website accordingly
* src/esx/esx_driver.c: set the auto generation prefix to 00:0c:29
* src/esx/esx_vmx.c: fix MAC address range handling and allow arbitrary MAC
  addresses
* tests/vmx2xml*, tests/xml2vmx*: add some basic MAC address range tests