qemu_command: Don't validate accelerator when building cmd line

qemu_command: Don't validate accelerator when building cmd line

The domain accelerator was validated in qemuValidateDomainDef()
which calls virQEMUCapsIsVirtTypeSupported() which reports proper
error if QEMU is not capable of KVM/TCG. There is no point in
doing the validation again when building command line.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>