To avoid regressions, we let callers specify whether to require a
minor and micro version.  Callers that were parsing uname() output
benefit from defaulting to 0, whereas callers that were parsing
version strings from other sources should not change in behavior.

* src/util/util.c (virParseVersionString): Allow caller to choose
whether to fail if minor or micro is missing.
* src/util/util.h (virParseVersionString): Update signature.
* src/esx/esx_driver.c (esxGetVersion): Update callers.
* src/lxc/lxc_driver.c (lxcVersion): Likewise.
* src/openvz/openvz_conf.c (openvzExtractVersionInfo): Likewise.
* src/uml/uml_driver.c (umlGetVersion): Likewise.
* src/vbox/vbox_MSCOMGlue.c (vboxLookupVersionInRegistry):
Likewise.
* src/vbox/vbox_tmpl.c (vboxExtractVersion): Likewise.
* src/vmware/vmware_conf.c (vmwareExtractVersion): Likewise.
* src/xenapi/xenapi_driver.c (xenapiGetVersion): Likewise.
Reported by Matthias Bolte.