bootstrap: Insert test provider name in subtests.cfg namespace

In order to make tests from 2 different providers,
but with the same name work, insert their provider
names inside subtests.cfg. Also, write the provider
name value in the cfg and restrict the search for
source names.

With this, we can tell whether a hypothetical 'ping'
in 'io-github-autotest-qemu' actually comes from that
provider and not, say, 'io-github-spiceqa-qemu'.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virt.py: Adapting main autotest entry point to recent changes

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
backends: Remove useless 'agnostic' backend dir

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>