esx: Simplify VI (vSphere) API and VMware product version handling

esx: Simplify VI (vSphere) API and VMware product version handling

Store version numbers in this format

version = 1000000 * major + 1000 * minor + micro

produced by virParseVersionString instead of dedicated enums.

Split the complex esxVI_ProductVersion enum into a simpler
esxVI_ProductLine enum and a product version number.

Relax API and product version number checks to accept everything that
is equal or greater than the supported minimum version. VMware ESX
went through 3 major versions and the vSphere API always stayed
backward compatible. This commit assumes that this will also be true
for future VMware ESX versions.

Also reword error messages in esxConnectTo* to say what was expected
and what was found instead (suggested by Richard W.M. Jones).