Commit ca10bb040fcf introduced a new test that fails to build
on at least some architectures:
commandtest.c: In function 'test25':
commandtest.c:1121:5: error: comparison is always true due to
limited range of data type [-Werror=type-limits]
if (rv >= 0) {
^
Change the type of 'rv' from char to int, which is the proper
return type for virCommandExec() anyway.