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: NLucas Meneghel Rodrigues <lmr@redhat.com>

qemu/cfg/build.cfg

@@ -42,6 +42,15 @@ variants:
         # use it to fetch object first from it, and then later from "upstream"
         # git_repo_qemu_base_uri = /home/user/code/qemu
 
+
+        # QEMU installation from a GIT repo (signed tags)
+        #git_repo_qemu_uri = git://git.kernel.org/pub/scm/virt/kvm/mst/qemu.git
+        #git_repo_qemu_configure_options = --target-list=x86_64-softmmu
+        #git_repo_qemu_tag = for_anthony
+        # If tag_signed is provided, then we will be strict and verify the
+        # tag. If the tag verification fails, the entire build test will fail.
+        #git_repo_qemu_tag_signed = pgp.mit.edu:AFBE8E67
+
         # SPICE installation from a GIT repo
         git_repo_spice_uri = git://anongit.freedesktop.org/spice/spice
         git_repo_spice_common_uri = git://anongit.freedesktop.org/spice/spice-common