.github: Make all release tags follow v<semver> convention

Replace xemu-v*, gh-release/* tagging with typical semver vX.Y.Z, which
plays nicer with GitHub releases. Increments patch version on push
to master.

scripts/increment-semver.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python3
+import argparse, re
+ap = argparse.ArgumentParser()
+ap.add_argument('semver', help='Input semver X.Y.Z')
+args = ap.parse_args()
+m = re.match(r'(?P<pfx>[^\d]*)(?P<maj>\d+)\.(?P<min>\d+)\.(?P<pat>\d+)',
+             args.semver)
+assert m, 'Invalid version format'
+print(m.group('pfx') + '.'.join([m.group('maj'), m.group('min'),
+                                str(int(m.group('pat')) + 1)]))