build: properly skip tests

build: properly skip tests

I got this failure on FreeBSD:

shunloadtest.c: In function 'main':
shunloadtest.c:150: error: 'EXIT_AM_SKIP' undeclared (first use in this function)

but inspection showed several other problems, all fixed here.

* tests/domainsnapshotxml2xmltest.c [!WITH_QEMU]: Ensure
EXIT_AM_SKIP is defined.
* tests/esxutilstest.c [!WITH_ESX]: Likewise.
* tests/openvzutilstest.c [!WITH_OPENVZ]: Likewise.
* tests/qemuargv2xmltest.c [!WITH_QEMU]: Likewise.
* tests/qemuhelptest.c [!WITH_QEMU]: Likewise.
* tests/qemuxml2argvtest.c [!WITH_QEMU]: Likewise.
* tests/qemuxml2xmltest.c [!WITH_QEMU]: Likewise.
* tests/qemuxmlnstest.c [!WITH_QEMU]: Likewise.
* tests/shunloadtest.c [!linux]: Likewise.
* tests/vmx2xmltest.c [!WITH_VMX]: Likewise.
* tests/xml2vmxtest.c [!WITH_VMX]: Likewise.