vpoly = QgsVectorLayer("Polygon", "pointbuffer", "memory")
feature = QgsFeature()
feature.setGeometry( QgsGeometry.fromPoint(QgsPoint(3517601,5406205)).buffer(10000,5))
provider = vpoly.dataProvider()
vpoly.startEditing()
provider.addFeatures( [feature] )
vpoly.commitChanges()

vpoly = QgsVectorLayer("Polygon", "pointbuffer", "memory")
feature = QgsFeature()
feature.setGeometry( QgsGeometry.fromPoint(QgsPoint(3517601,5406205)).buffer(10000,5))
provider = vpoly.dataProvider()
vpoly.startEditing()
provider.addFeatures( [feature] )
vpoly.commitChanges()

def circle_geometry(self, pt, radius=0, segments=0, mapunits=False):
    """
    Draw a circle at a canvas point

    :type pt: qgis.core.QgsPoint
    :param pt: canvas point, in layer crs
    :type radius: float
    :param radius: cicle radius, considered to be in layer units
    :type segments: int
    :param segments: number of segments usually divisible by 8, e.g. 32
    :type mapunits: bool
    :param mapunits: whether the radius should be considered in map units
    :return: QgsGeometry of type QGis.Polygon
    """
    if not radius:
        radius = self.circleradius  # default of 5 for this project
    if not mapunits:
        ctx = self.canvas.mapRenderer().rendererContext()
        # as mm (converted to map pixels, then to map units)
        radius *= ctx.scaleFactor() * ctx.mapToPixel().mapUnitsPerPixel()
    if not segments:
        segments = self.circlesegments  # usually divisible by 8, e.g. 32
    pts = []
    for i in range(segments):
        theta = i * (2.0 * math.pi / segments)
        p = QgsPoint(pt.x() + radius * math.cos(theta),
                     pt.y() + radius * math.sin(theta))
        pts.append(p)
    return QgsGeometry.fromPolygon([pts])