Add some tests for options in qemu.conf

Test clear_emulator_capabilities parameter in qemu.conf.

1) Change clear_emulator_capabilities in qemu.conf;
2) Restart libvirt daemon;
3) Check if libvirtd successfully started;
4) Check if qemu process capabilities changed accordingly;

Test set_process_name parameter in qemu.conf.

1) Change set_process_name in qemu.conf;
2) Restart libvirt daemon;
3) Check if libvirtd successfully started;
4) Check if qemu command line changed accordingly;

Test seccomp_sandbox parameter in qemu.conf.

1) Change seccomp_sandbox in qemu.conf;
2) Restart libvirt daemon;
3) Check if libvirtd successfully started;
4) Check if qemu command line changed accordingly;

Signed-off-by: Hao Liu <hliu@redhat.com>
Although I never run into this line, I add this for precaution. I thought the most probable cause of this raise is the absence of capabilities support on the host, which is a environment mismatch and should raise a `TestNAError`. @will-Do, Do you agree on this?