qemu: Refactor lookup of domain object

Use the helper to lookup the domain object in the remaining places.

This patch also fixes error reporting when the domain was not found in several
functions that were printing the raw UUID buffer instead of the formatted
string. The offending functions were:

qemuDomainGetInterfaceParameters
qemuDomainSetInterfaceParameters
qemuGetSchedulerParametersFlags
qemuSetSchedulerParametersFlags
qemuDomainGetNumaParameters
qemuDomainSetNumaParameters
qemuDomainGetMemoryParameters
qemuDomainSetMemoryParameters
qemuDomainGetBlkioParameters
qemuDomainSetBlkioParameters
qemuDomainGetCPUStats