@def->clock validation is done by qemuBuildClockCommandLine() and
qemuBuildClockArgStr(). This patch centralize the validation done
in both these functions to a new qemuDomainDefValidateClockTimers()
function. This new function is then called by qemuDomainDefValidate(),
promoting clock validation in domain define time.

Tests were adapted to consider the new caps being needed in
this earlier stage.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>