virttest.build_helper: Introduce tag checkout and tag sign verification

Sometimes we want to check qemu from a git tag, that allows
us to verify whether the code we are downloading actually
comes from the repository owner.

Introduce this support to our git helper. If for some reason
this verification fails, be merciless and fail the entire
build test.

CC: Michael S. Tirskin <mst@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>