qemu: command: move virDomainClockDef validation to qemu_domain.c

qemu: command: move virDomainClockDef validation to qemu_domain.c

@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: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>