qemuxml2argvtest: Run some test only on Linux

qemuxml2argvtest: Run some test only on Linux

As I was reviewing bhyve commits, I've noticed qemuxml2argvtest
failing for some test cases. This is not bug in qemu driver code
rather than being unable to load qemuxml2argvmock on non-Linux
platforms. For instance:

318) QEMU XML-2-ARGV numatune-memnode
... libvirt:  error : internal error: NUMA node 0 is unavailable
FAILED

Rather than disabling qemuxml2argvtest on BSD (we do compile qemu
driver there) disable only those test cases which require mocking.
To achieve that goal new DO_TEST_LINUX() macro is introduced which
invokes the test case on Linux only and consume arguments on other
systems.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>