Coverity complained that most, but not all, clients of virUUIDParse
were checking for errors.  Silence those coverity warnings by
explicitly marking the cases where we trust the input, and fixing
one instance that really should have been checking.  In particular,
this silences a rather large percentage of the warnings I saw on my
most recent Coverity analysis run.

* src/util/uuid.h (virUUIDParse): Enforce rules.
* src/util/uuid.c (virUUIDParse): Drop impossible check; at least
Coverity will detect if we break rules and pass NULL.
* src/xenapi/xenapi_driver.c (xenapiDomainCreateXML)
(xenapiDomainLookupByID, xenapiDomainLookupByName)
(xenapiDomainDefineXML): Ignore return when we trust data source.
* src/vbox/vbox_tmpl.c (nsIDtoChar, vboxIIDToUUID_v3_x)
(vboxCallbackOnMachineStateChange)
(vboxCallbackOnMachineRegistered, vboxStoragePoolLookupByName):
Likewise.
* src/node_device/node_device_hal.c (gather_system_cap): Likewise.
* src/xenxs/xen_sxpr.c (xenParseSxpr): Check for errors.