env_process: Check if VM exists when preprocessing

Before this patch when running test cases without a main VM exists,
say `virt-tests-vm1`, the test still goes on, it will raise a `TestFail`
if it tries to start the non-exist VM or if test was configured not to be
started at the preprocess progress, there will be strange outputs.

This patch check the existence of target VM, and raise a `TestError` if
`main_vm` don't exist or a `TestNAError` if a non-`main_vm` don't exist.

Signed-off-by: Hao Liu <hliu@redhat.com>